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AGRICULTURAL    ENGINEERING    SERIES 
E.    B.    McCORMICK,    Consulting    Editor 

MECHANICAL    ENGINEER,    OFFICE    OF   PUBLIC   ROADS 

U.    S.    DEPARTMENT   OF   AGRICULTURE 

FORMERLY   DEAN   OF   ENGINEERING   DIVISION 

KANSAS   STATE   AGRICULTURAL  COLLEGE 


CONCRETE   CONSTRUCTION 
FOR   RURAL   COMMUNITIES 


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— — 


CONCRETE    CONSTRUCTION 

FOR 

RURAL  COMMUNITIES 


BY 

ROY  A.  SEATON,  M.S. 


PROFESSOR    OF    APPLIED    MECHANICS    AND    MACHINE    DESIGN    AND    SUPERINTENDENT    OF 

CONSTRUCTION,   KANSAS   STATE   AGRICULTURAL   COLLEGE;    MEMBER   OF    AMERICAN 

SOCIETY   FOR  TESTING    MATERIALS,   AND   SOCIETY   FOR   THE    PROMOTION 

OF    ENGINEERING     EDUCATION;      ASSOCIATE    MEMBER     OF 

AMERICAN  SOCIETY  OF   MECHANICAL  ENGINEERS 


Fisst  •EmTtfW 


McGRAW-HILL  BOOK  COMPANY  Inc. 
239  WEST  39TH  STREET,  NEW  YORK 

6  BOUVERIE  STREET,  LONDON,  E.C. 

1916 


Copyright,   1916,  by  the 
McGraw-Hill  Book  Company,  Inc. 


PREFACE 

The  extension  of  the  use  of  concrete  to  farms  and  rural  communi- 
ties has  developed  a  need  for  a  textbook  treating  the  essential 
features  of  concrete  construction  in  a  thorough  but  simple  manner. 
Most  of  the  works  now  available  either  are  written  for  engineers  and 
are  unsuited  to  the  non-technical  man,  or  are  in  the  form  of  bul- 
letins dealing  chiefly  with  the  uses  to  which  concrete  is  put  in  rural 
communities,  without  giving  a  systematic  treatment  of  fundamental 
principles  governing  its  use.  The  author  has  endeavored,  in  writing 
this  book,  to  make  it  suitable  for  use  as  a  text  in  a  brief  course  in  con- 
crete construction  for  agricultural  or  other  students  in  colleges,  when 
accompanied  by  laboratory  exercises  and  field  construction,  and  to 
make  it  valuable  to  others  who  have  occasion  to  use  concrete. 
Chapter  II,  on  Cement  Specifications  and  Tests,  and  Chapter  VIII, 
on  Strength  of  Reinforced  Concrete,  are  necessarily  somewhat  techni- 
cal, but  it  is  thought  best  to  include  this  matter  for  the  use  of  students. 
Persons  who  find  the  matter  difficult  or  uninteresting  may  pass  over 
these  chapters. 

In  the  preparation  of  the  book,  various  sources  of  information  have 
been  freely  used,  chief  among  which  are  Taylor  and  Thompson's 
"Concrete,  Plain  and  Reinforced,"  and  the  bulletins  published  by 
the  Universal  Portland  Cement  Company,  of  Chicago,  The  Atlas 
Portland  Cement  Company  of  New  York,  the  Association  of  Amer- 
ican Portland  Cement  Manufacturers  of  Philadelphia,  and  the  Office 
of  Public  Roads  and  Rural  Engineering,  United  States  Department 
of  Agriculture,  Washington,  D.  C. 

The  author  wishes  to  express  his  gratitude  to  those  who  have 
given  him  permission  to  use  their  tables  and  figures  and  to  acknowl- 
edge his  indebtedness  to  Professors  N.  A.  Crawford,  P.  J.  Freeman, 
and  M.  R.  Bowerman  of  the  Kansas  State  Agricultural  College  for 
assistance  in  the  preparation  of  the  manuscript. 

Manhattan,  Kansas.  R.  A.  SEATON 

February,  1916 

33807  6 


CONTENTS 

PAGE 

Preface vii 

Introduction 1 

Definitions.  —  Development  of  the  use  of  concrete 

PART   I  — MATERIALS 

CHAPTER  I 

Cements  and  Limes 7 

Kinds  of  cements.  —  Portland  cement.  —  Natural  cement.  —  Puz- 
zolan,  or  slag,  cement.  —  Lime.  —  Hydrated  lime.  —  Choice  of 
a  cement.  —  Care  of  cement. 

CHAPTER  II 

Cement  Specifications  and  Tests 13 

For  the  small  purchaser.  —  For  larger  and  more  important  work.  — 
Specifications  of  American  Society  for  Testing  Materials.  —  Meth- 
ods for  testing  as  prescribed  by  American  Society  of  Civil  Engineers. 

—  Soundness  test  without  special  apparatus. 

CHAPTER  III 
Aggregates 34 

Aggregates.  —  Selection  of  aggregates.  —  Sand.  —  Requirements  of 
sand.  —  Use  of  find  sand.  —  Uniformity  coefficient.  —  Impurities.  — 
Test  for-impurities.  —  Organic  impurities.  —  Strength  test  for  sand. 

—  Stone  screenings.  —  Broken  stone.  —  Size  of  stone.  —  Gravel.  — 
Washing  of  gravel.  —  Cinders. 

PART  II  — PLAIN   CONCRETE 
CHAPTER  IV 

Proportions  and  Quantities  of  Materials 47 

The  problem  in  proportioning  concrete.  —  Voids.  —  To  find  the  per- 
centage of  voids  in  coarse  aggregate.  —  To  find  the  percentage  of 
voids  in  fine  aggregate.  —  Proportions  for  maximum  density.  — 
Fundamental  laws  of  proportioning.  —  Arbitrarily  specified  propor- 
tions. —  Other  methods  of  proportioning.  —  Trial  mixtures.  — 
Mechanical  analysis.  —  Omission  of  coarse  aggregate.  —  Use  of 
bank  run  gravel.  —  Quantities  of  materials  required.  —  Fuller's  Rule. 

—  Table  of  quantities.  —  Problems. 


X  CONTENTS 

CHAPTER  V  page 

Construction  of  Forms 62 

Materials  used  for  forms.  —  Use  of  wood  for  forms.  —  Tying  and 
bracing  forms.  —  Forms  for  straight  walls.  —  Forms  for  circular 
walls.  —  Forms  for  overhead  floors  and  roofs.  —  Column  forms.  — 
Time  of  removal  of  forms. 

CHAPTER  VI 

Mixing  and  Handling  Concrete 71 

Requirements  of  Good  Mixing.  —  Consistency.  —  Methods  of 
Mixing.  —  Tools  required  for  hand  mixing.  —  Measuring  the  mate- 
rials.—  Mixing  the  materials  by  hand.. —  Machine  mixing. — 
Continuous  mixers.  —  Batch  mixers.  —  Transporting  concrete.  — 
Depositing  concrete.  —  Curing  of  concrete.  —  Bonding  old  and  new 
concrete.  —  Contraction  and  expansion  joints.  —  Setting  of  con- 
crete. —  Strength  of  Concrete.  —  Effect  of  freezing  on  concrete. 

PART  III  — REINFORCED  CONCRETE 

CHAPTER  VII 
General  Principles 95 

Necessity  for  reinforcing.  —  Materials  used  for  reinforcement.  — 
Grades  of  steel  used.  —  Specifications  for  reinforcing  steel.  —  Forms 
of  reinforcing  steel.  —  Classes  of  structures  in  which  reinforced  con- 
crete is  used.  —  Hollow  cylinders  subjected  to  internal  pressure.  — 
Placing  the  steel.  —  Beams  and  slabs.  —  Continuous  beams.  — 
Depth  of  embedment  of  steel.  —  Columns.  —  Arches  and  hollow  cyl- 
inders subjected  to  external  pressure.  —  Differences  in  materials 
and  methods  used  for  reinforced  concrete. 

CHAPTER  VIII 

Strength  of  Reinforced  Concrete Ill 

Stresses  used  in  reinforced  concrete.  —  Hollow  cylinders  subjected 
to  internal  pressure.  —  Columns.  —  Beams  and  slabs.  —  Rectangular 
beams.  —  Floor  and  roof  slabs.  —  Continuous  beams.  —  T-b'eams.  — 
Arches  and  hollow  cylinders  subjected  to  external  pressure.  — 
Problems. 

PART   IV  — MISCELLANEOUS   MATTERS 

CHAPTER  IX 

Concrete  Surface  Finishes 125 

Methods  of  treatment.  —  Mortar  face.  —  Brushed  surface.  — 
Scrubbed  or  etched  surface.  —  Tooled  surface.  —  Plastered  surface. 

CHAPTER  X 
Stucco  and  Plaster  Work 131 

Uses  of  stucco.  —  Stucco  surface  finishes.  —  Specifications  for  stucco 


CONTENTS  xi 

CHAPTER  XI  pAGE 

Waterproofing  and  Coloring  Concrete 142 

Necessity  for  waterproofing.  —  Precautions  to  be  observed  in  water- 
tight work.  —  Methods  of  waterproofing.  —  Use  of  hydrated  lime. 
—  Use  of  water  repellent  compounds.  —  Surface  coatings.  — 
Bituminous  shield.  —  Coloring  concrete.  —  Colored  aggregates.  — 
Mineral  pigments.  —  Painting  concrete  surfaces. 

CHAPTER  XII 

Casting  in  Molds 150 

Building  blocks.  —  Processes  of  manufacture.  —  Block  machines.  — 
Kinds  of  blocks.  —  Waterproofing  and  coloring  of  blocks.  —  Curing  of 
blocks.  —  Cost  of  concrete  blocks.  —  Specifications  for  concrete 
blocks.  —  Concrete  brick.  —  Building  trim.  —  Drain  tile.  —  Rein- 
forced concrete  pipe.  —  Concrete  fence  posts.  —  Corner  and  gate 
posts.  —  Fastening  fence  to  posts.  —  Cost  of  posts. 

PART  V  — TYPICAL  APPLICATIONS  OF  CONCRETE 

CHAPTER  XIII 
Sidewalks,  Floors,  and  Roads 177 

Sidewalks.  —  Preparation  of  subgrade.  —  Proportions.  —  Forms.  — 
Mixing  and  placing  the  concrete.  —  Surface  finish.  —  Curing.  —  Cost  of 
sidewalks.  —  Use  of  concrete  for  floors.  —  Cellar  floors.  —  Barn 
Floors.  —  Feeding  floors.  —  Concrete  roads.  —  Construction  of 
roads.  —  Cost  of  Roads. 

CHAPTER  XIV 

Tanks,  Cisterns,  and  Silos      188 

Use  of  concrete  for  water  tanks.  —  Construction  of  tanks.  —  Rein- 
forcing. —  Cisterns.  —  Requirements  of  a  good  silo.  —  Use  of  con- 
crete for  silos.  — Size  of  silo  required.  —  Forms.  —  Construction  of 
the  silo.  —  Doors  and  chute.  —  Reinforcing.  —  Roofs.  —  Cost  of 
silos. 

CHAPTER  XV 

Small  Highway  Bridges  and  Culverts 206 

Use  of  concrete  for  bridges  and  culverts.  —  Types  of  bridges.  — 
Size  of  bridge.  —  Foundations.  —  Abutments.  —  Flat  slab  bridges. 
Flat  top  box  culverts.  —  Circular  top  box  culverts. 

Index 221 


CONCRETE   CONSTRUCTION   FOR 
RURAL   COMMUNITIES 


INTRODUCTION 

Definitions. —  Concrete  is  an  artificial  stone  made  by  cementing 
together  particles  of  sand,  gravel,  broken  stone,  or  other  hard, 
inert  substances,  designated  as  the  aggregate.  Usually  a  cement 
is  used  which  requires  only  the  addition  of  water  to  make  it 
harden,  or  set,  and  only  concretes  of  this  class  will  be  considered 
in  this  book.  The  term  cement  is  frequently  applied  to  such 
objects  as  walks,  building  blocks,  and  silos,  especially  when  they 
do  not  contain  coarse  particles  of  stone  or  gravel,  but  they 
should  properly  be  called  concrete  walks,  concrete  building  blocks, 
or  concrete  silos,  and  the  name  cement  should  be  restricted  to  the 
material  used  to  bind  the  aggregate  together. 

Mortar  is  a  name  often  given  to  concretes  containing  no 
coarse  aggregate,  or  to  that  part  of  concrete  which  consists  of 
cement,  sand,  and  water.  The  name  is  almost  universally  ap- 
plied to  mixtures  —  especially  before  they  have  hardened  —  used 
for  filling  the  joints  of  masonry,  and  for  plastering. 

Development  of  the  Use  of  Concrete. — Concrete  was  used  for 
construction  work  by  ancient  peoples  many  centuries  before 
Christ.  The  Egyptians  and  the  Carthaginians  are  said  to  have 
used  it  to  some  extent,  while  the  Romans  employed  it  extensively 
in  foundations,  walls,  and  aqueducts.  Their  concrete  was  made 
by  cementing  together  sand  and  broken  stones  with  a  mixture  of 
volcanic  ash  and  slaked  lime.  The  cement  resulting  from  this 
mixture  was  similar  to  the  puzzolan,  or  slag,  cement  manufac- 
tured at  the  present  time.  Some  of  the  structures  built  by  the 
Romans  are  still  in  a  good  state  of  preservation  after  standing 
for  two  thousand  years,  thus  testifying  to  the  permanence  of 
concrete  construction. 

1 


2       CONCRETE  CONSTRUCTION  FOR  RURAL  COMMUNITIES 

During  the  Middle  Ages  the  use  of  concrete  was  continued  to 
some  extent,  but  no  great  advance  was  made.  In  the  eighteenth 
century  it  was  discovered  that  the  clay  contained  in  certain  limes 
conferred  on  them  the  property  of  setting,  or  hardening,  under 
water,  and  the  process  of  making  natural  cement  was  developed. 

In  the  last  century  the  use  of  concrete  was  greatly  stimulated 
by  the  invention  of  Portland  cement,  by  the  discovery  of  de- 
posits of  rock  suitable  for  making  natural  cement,  and  by  the 
development  of  machinery  which  made  it  possible  to  produce 
these  cements  cheaply. 


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Year 

Fig.  1.  —  Production  of  Portland  and  Natural  Cements  in  the  United  States 

by  Years. 

In  the  earlier  part  of  the  last  century,  most  of  the  cement  used 
in  this  country  was  imported.  The  first  discovery  of  rock  in  the 
United  States  suitable  for  making  natural  cement  was  made  in 
1818.  The  first  Portland  cement  produced  in  the  United  States 
was  made  by  D.  O.  Saylor  in  the  Lehigh  Valley  of  Pennsylvania 
about  1875. 

The  present  century  has  seen  a  wonderful  growth  in  the  use  of 
concrete  in  this  country.  The  decreasing  cost  of  cement  and  an 
increasing  appreciation  of  the  valuable  properties  of  concrete 
have  made  its  use  grow  enormously.     Figure   1,  showing  the 


INTRODUCTION  3 

production  of  Portland  and  natural  cements  in  the  United 
States,  illustrates  the  phenomenal  growth  of  the  industry.  New 
uses  for  this  material  have  been  discovered  constantly,  and  its 
field  has  been  widened  until  now  it  is  used  in  all  kinds  of  construc- 
tion work.  In  the  cities,  large  buildings  are  made  almost  entirely 
of  concrete  and  steel.  Foundations,  walls,  columns,  floors, 
and  roofs  are  all  made  of  concrete,  most  of  which  is  reinforced  by 
steel  rods.  Roads,  pavements,  bridges,  dams,  tunnels,  sewers,  and 
reservoirs  are  now  built  of  concrete.  Its  use  is  not  confined  to 
the  cities  and  to  large  construction  work,  however,  but  extends 
to  smaller  towns  and  to  farms  as  well.  The  most  progressive 
farmers  use  it  for  foundations,  sidewalks,  basement  floors, 
feeding  floors,  root  cellars,  buildings,  water  tanks,  silos,  fence 
posts,  and  numerous  other  structures.  Its  value  for  such 
purposes  is  only  beginning  to  be  realized  and  it  is  reasonable  to 
believe  that  its  use  on  the  farm  will  in  the  near  future  be  greatly 
extended. 

The  low  cost  of  cement,  the  wide  distribution  of  sand,  stone, 
and  gravel  suitable  for  use  in  concrete,  the  ease  with  which  it 
can  be  manufactured  and  molded  to  any  desired  form,  and  the 
permanence  of  concrete  structures  when  properly  made,  to- 
gether with  the  fact  that  no  skilled  labor  or  expensive  ma- 
chinery is  required  for  its  production,  make  concrete  one  of  the 
cheapest  and  best  of  structural  materials  in  use  at  the  present 
time,  and  make  it  almost  universally  available.  It  is  greatly  to 
be  desired  that  a  knowledge  of  the  valuable  properties  and  the 
methods  of  use  of  this  material  be  extended  as  widely  as  possible, 
in  order  that  the  tremendous  benefits  resulting  from  its  use  may 
be  generally  enjoyed. 


PART  I 
MATERIALS 


CHAPTER  I 

CEMENTS    AND    LIMES 

Kinds  of  Cements.  —  The  cements  chiefly  used  in  concrete  con- 
struction may  be  classified  as  follows: 

(a)  Portland  cement, 

(b)  Natural  cement, 

(c)  Puzzolan,  or  slag  cement. 

Of  these,  Portland  cement  is  by  far  the  most  widely  used. 
The  relative  importance  of  the  different  kinds  in  the  concrete, 
industry  of  the  United  States  is  indicated  by  the  fact  that  in 
1910  there  were  produced  in  this  country: 

76,550,000  barrels  of  Portland  cement, 
1,100,000  barrels  of  natural  cement,  and 
160,000  barrels  of  puzzolan  cement. 

It  is  seen  that  the  amount  of  Portland  cement  produced  was 
about  seventy  times  the  amount  of  natural  cement  and  almost 
five  hundred  times  the  amount  of  puzzolan  cement.  This 
being  the  case,  this  book  will  deal  almost  entirely  with  the  use 
of  Portland  cement,  the  other  kinds  being  treated  only  inci- 
dentally. Throughout  the  book,  therefore,  wherever  the  word 
cement  is  used,  it  should  be  understood  to  refer  only  to  Port- 
land cement,  unless  it  is  explicitly  stated  otherwise. 

Portland  Cement.  —  Portland  cement  is  defined  by  the  Ameri- 
can Society  for  Testing  Materials  as  "the  finely  pulverized 
product  resulting  from  the  calcination  to  incipient  fusion  of 
an  intimate  mixture  of  properly  proportioned  argillaceous  and 
calcareous  materials,  and  to  which  no  addition  greater  than 
3  per  cent  has  been  made  subsequent  to  calcination."  This  is 
equivalent  to  saying  that  it  is  the  product  obtained  when  clay 
and  limestone,  or  materials  of  similar  chemical  composition,  are 
ground    together   in   the    proper   proportion,    are   then    heated 

7 


8     CONCRETE -CONSTRUCTION  FOR  RURAL  COMMUNITIES 

nearly  to  the  melting  pomt,  and  the  clinker  thus  produced  is 
ground  to  a  fine  powder.  The  clause  limiting  additions  to  the 
material  after  burning  to  3  per  cent  is  designed  to  prevent 
adulteration,  but  to  permit  of  the  addition  of  a  small  amount 
of  gypsum  or  plaster  of  Paris  to  regulate  the  rate  of  setting  of 
the  cement.  Usually  1  or  2  per  cent  of  gypsum  is  required 
for  this  purpose.  It  is  added  to  the  clinker  after  the  burning, 
but  before  the  final  grinding. 

As  may  be  seen  from  the  above  definition,  the  name  Port- 
land does  not  refer  to  cements  made  in  any  particular  locality. 
It  was  given  to  the  material  by  the  inventor,  because  of  the 
fancied  resemblance  of  concrete  made  from  it  to  the  building 
stone  quarried  on  the  Isle  of  Portland,  England. 

The  essential  materials  for  making  Portland  cement  are  cal- 
cium carbonate,  the  principal  constituent  of  limestone;  and 
aluminum  silicate,  the  principal  constituent  of  clay.  These 
materials  are  usually  used  in  the  proportion  of  about  3  parts  of 
the  former  to  1  part  of  the  latter.  They  occur  naturally  in 
many  different  forms  and  are  very  commonly  found  mixed 
together.  In  a  few  localities  stone  is  found  which  contains 
them  already  mixed  in  approximately  correct  proportions. 
More  commonly  an  excess  of  one  of  the  materials  is  present, 
and  the  deficiency  of  the  other  must  be  made  up  by  the  addi- 
tion of  some  substance  in  which  it  occurs  to  excess.  In  the 
United  States,  the  materials  used  to  supply  the  calcium  carbon- 
ate are  chiefly  limestone,  chalk,  or  marl,  while  the  aluminum 
silicate  is  supplied  by  clay,  shale,  blast-furnace  slag,  or  cement 
rock. 

Suitable  materials  for  the  manufacture  of  Portland  cement 
are  widely  distributed.  More  than  half  of  the  states  in  the 
United  States  have  one  or  more  plants  for  its  manufacture,  and 
it  is  lack  of  cheap  fuel,  of  transportation  facilities,  or  of  a 
convenient  market,  rather  than  of  suitable  materials,  that  pre- 
vents its  manufacture  in  other  states.  Table  I  gives  the  pro- 
duction of  Portland  cement  in  the  United  States,  by  states,  in 
1911. 

The  color  of  Portland  cement  is  usually  a  bluish  gray,  though 
the  tint  differs  considerably  in  different  brands.     Certain  mills 


CEMENT  AND  LIMES 


0 


make  a  special  kind  of  Portland  cement  which  is  almost  pure 
white  in  color,  and  which  is  much  used  for  ornamental  purposes. 

Table  I 

Production  of  Portland  Cement  in  the  United  States  by  States 

in  1911 


States 


Pennsylvania. 

Indiana 

California.  .  .  . 

Kansas 

Illinois 

New  Jersey .  . 
Missouri 
Michigan 
New  York .  . . 

Iowa 

Ohio 

Washington  . 

Utah 

Texas 

Oklahoma.  .  . 
Tennessee 
West  Virginia 
Kentucky 

Virginia 

Maryland 
Colorado.  .  .  . 
Montana.  .  .  . 

Alabama 

Georgia 

Total 


Producing 
plants 


25 
5 
8 

12 
5 
3 
4 

11 
7 
3 
5 
3 
3 
4 
2 
2 
1 
1 
2 
2 
2 
1 
2 
2 


115 


Barrels 


26,864,679 
7,407,830 
6,317,701 
4,871,903 
4,582,341 
4,411,890 
4,114,859 
3,686,716 
3,314,217 
1,952,590 
1,451,852 
960,573 
662,849 

2,438,493 

1,981,341 

1,487,753 

1,162,081 

858,969 


78,528,637 


Value 


$19,258,253 
5,937,241 
8,737,150 
3,725,108 
3,583,301 
3,259,528 
3,349,312 
3,024,676 
2,669,194 
1,881,253 
1,228,680 
1,496,807 
827,523 

2,541,449 

1,590,438 

1,084,315 
1,272,317 

782,272 


$66,248,817 


Natural  Cement.  —  Natural  cement  is  defined  by  the  American 
Society  for  Testing  Materials  as  "the  finely  pulverized  product 
resulting  from  the  calcination  l  of  an  argillaceous 2  limestone  at 
a  temperature  only  sufficient  to  drive  off  the  carbonic  acid 
gas."  It  therefore  differs  from  Portland  cement  in  the  follow- 
ing respects:  (1)  only  one  material  is  used  in  its  manufacture, 
a  clayey  limestone  of  more  or  less  varying  composition,  instead 
of  a  definitely  proportioned  mixture  of  clay  and  limestone;  (2) 
this  rock  is  not  pulverized  before  being  burned;  (3)  the  tem- 
perature of  burning  is  very  much  lower.     The  resulting  prod- 

1  That  is,  burning.  2  That  is,  clayey. 


10     CONCRETE  CONSTRUCTION  FOR   RURAL  COMMUNITIES 

uct  is  much  inferior  to  Portland  cement  in  cementing  power. 
On  account  of  the  simpler  process  of  manufacture,  natural 
cement  can  be  sold  more  cheaply  than  Portland  cement.  It 
can  often  be  distinguished  from  the  latter  by  its  lighter  color, 
but  this  is  not  a  positive  indication,  as  some  natural  cements 
are  dark,  while  some  Portland  cements  are  light  in  color. 

Natural  cement  was  formerly  of  considerable  importance, 
but  its  use  is  decreasing  and  its  place  is  being  taken  by  Port- 
land cement,  as  may  be  seen  by  reference  to  Fig.  1.  While 
the  latter  costs  more  per  barrel  than  the  former,  much  less  of  it 
is  required  to  produce  a  concrete  of  the  same  strength.  Its  use 
is  therefore  more  economical,  except  perhaps  in  certain  locali- 
ties or  for  certain  special  uses. 

Puzzolan,  or  Slag  Cement.  —  Puzzolan,  or  slag  cement,  is 
formed  by  mixing  granulated  blast-furnace  slag  of  special  com- 
position with  slaked  lime,  and  grinding  the  mixture  to  a  very 
fine  powder.  In  Europe  volcanic  ash  is  sometimes  used  instead 
of  the  blast-furnace  slag. 

This  cement  differs  from  Portland  cement  made  from  the  same 
materials,  in  that  it  is  not  burned  after  its  constituents  are 
mixed,  and  in  that  its  properties  are  considerably  different. 
While  it  may  give  about  the  same  strength  to  mortars  and  con- 
cretes as  Portland  cement,  concrete  made  with  it  is  more  likely 
to  disintegrate  or  crumble  to  pieces,  especially  in  dry  air,  than 
that  made  with  Portland  cement. 

Lime.  —  When  a  pure  or  nearly  pure  limestone  is  heated 
sufficiently,  the  carbon  dioxide  is  driven  off,  and  calcium  oxide 
remains  as  a  white,  hard  stone.  This  forms  the  quicklime  of 
commerce.  It  has  a  strong  affinity  for  water  and,  when  brought 
in  contact  with  the  latter,  it  will  absorb  a  large  quantity.  At 
the  same  time,  the  lime  swells  in  volume  to  two  or  three  times 
its  original  bulk,  the  pieces  break  up  and  form  a  fine  powder, 
or,  if  enough  water  is  added,  a  smooth  paste,  and  a  large  amount 
of  heat  is  given  off.  This  process  is  known  as  slaking,  and  the 
product  is  called  slaked  lime,  lime  putty,  or  lime  paste. 

When  lime  paste  is  exposed  to  the  air  for  some  time,  it 
hardens.  This  action  is  due  to  three  distinct  causes:  (1)  drying 
of  the  paste;    (2)  crystallization  of  the  dissolved  lime;    and  (3) 


CEMENT  AND  LIMES  11 

absorption  of  carbon  dioxide  from  the  air,  which  changes  the 
mass  to  calcium  carbonate,  similar  to  the  original  limestone. 
The  last  mentioned  action  goes  on  but  slowly,  and  continues  for  a 
very  long  time.  It  takes  place  first  and  chiefly  near  the  surface, 
and  the  interior  of  large  masses  never  is  completely  changed. 

Hydrated  Lime.  —  Hydrated  lime  is  lime  which  has  been  slaked 
at  the  factory  by  the  addition  of  just  enough  water  to  produce 
a  fine,  impalpable  powder.  It  is  being  marketed  in  large  quan- 
tities. Its  advantages  are  that  it  is  in  powder  form  and  is  com- 
pletely slaked  and  ready  for  use.  It  is  much  more  conveniently 
used  in  concrete  work  than  quicklime,  as  its  powdered  form 
makes  it  easy  for  the  workman  to  obtain  proper  proportions 
and  to  get  the  lime  uniformly  distributed  throughout  the  mass. 

Choice  of  a  Cement.  —  Portland  cement  is  well  adapted  for  all 
kinds  of  concrete  construction,  and  is  usually  to  be  chosen  in 
preference  to  any  other  kind.  It  is  recommended  that  it  be 
used  for  all  purposes  by  those  not  thoroughly  familiar  with 
concrete  construction,  and  throughout  this  book  all  the  propor- 
tions indicated,  and  all  the  designs  given,  are  intended  for  use 
only  with  Portland  cement.  It  is  especially  important  that  it 
be  used  in  all  places  where  high  strength,  water-tightness,  or 
resistance  to  abrasion  is  required. 

Natural  cement  may  be  used  for  concrete  where  but  little 
strength  is  required,  and  where  it  will  not  be  subjected  to 
wear  or  to  the  action  of  water.  It  should  never  be  used  in 
reinforced  concrete  work,  nor  in  moist  situations.  Its  use  for  any 
purpose  is  not  recommended  unless  it  is  decidedly  more  economi- 
cal than  Portland  cement  after  one  has  taken  into  account  the 
fact  that,  with  the  latter,  a  much  leaner  mixture  may  be  used. 

Puzzolan  cement  may  be  used  for  concrete  in  moist  or  wet 
situations,  but  it  is  likely  to  crumble  when  exposed  to  dry  air 
for  any  considerable  length  of  time.  It  usually  gives  some- 
what weaker  concretes  than  Portland  cement,  used  in  the  same 
proportions,  and  this  should  be  taken  into  account  in  consider- 
ing the  relative  economy  of  the  two. 

The  choice  of  the  brand  of  cement  to  be  used  on  small  jobs 
must  usually  be  based  on  the  reputations  which  the  different 
brands  bear  in   the  particular  locality  in  question.     An  intelli- 


12     CONCRETE  CONSTRUCTION  FOR  RURAL  COMMUNITIES 

gent  choice  between  brands  made  by  reputable  manufacturers 
can  usually  be  made  only  after  tests  which  are  much  too  elabo- 
rate to  be  performed  by  those  unfamiliar  with  cement  testing 
and  unequipped  with  apparatus.  These  tests  are  described  in 
the  next  chapter.  Any  brand  which  meets  the  standard  speci- 
fications of  the  American  Society  for  Testing  Materials  may  be 
used  with  confidence,  and  all  reputable  manufacturers  now 
guarantee  their  products  to  meet  these  specifications.  Gener- 
ally it  is  sufficient  for  the  small  user  to  buy  a  brand  which  he 
knows  has  given  satisfaction  on  other  work  in  his  locality  and 
to  see  that  the  cement  is  not  hard  or  lumpy.  If  desired,  the 
soundness,  or  constancy  of  volume,  test  may  be  made  as  de- 
scribed at  the  end  of  the  next  chapter. 

Care  of  Cement.  —  Cement  is  usually  packed  in  cloth  bags, 
each  containing  94  pounds  of  cement.  Occasionally  it  is  packed 
in  wooden  barrels  or  in  paper  bags,  or  is  shipped  in  bulk.  The 
cloth  bags  are  charged  for  when  the  cement  is  bought,  but  a 
full  rebate  is  given  for  sacks  returned  in  good  condition,  so 
that  these  make  economical  packages.  The  cloth  bag  is  much 
more  convenient  to  handle  than  the  wooden  barrel,  and  not  so 
liable  to  breakage  as  the  paper  bag.  Consequently  it  is  replac- 
ing these  forms  of  packages.  Bulk  shipments  are  not  made  at 
present  to  any  considerable  extent. 

It  is  important  that  cement  be  stored  in  a  dry  place.  If  it 
is  piled  directly  on  the  earth,  or  on  a  floor  laid  on  the  earth,  or 
if  it  is  piled  against  an  outside  wall  for  any  length  of  time,  it 
will  absorb  moisture  and  will  become  hard  and  lumpy,  even 
though  it  is  kept  from  direct  contact  with  water.  A  false  floor 
should  be  made  of  boards  laid  on  blocks,  to  separate  the  cement 
from  the  floor  and  to  allow  a  circulation  of  air  beneath  it,  and 
the  sacks  should  be  piled  a  few  inches  away  from  all  outside 
walls.  Much  cement  has  been  ruined  by  failure  to  observe 
these  precautions.  When  well  protected,  cement  can  be  stored 
for  an  indefinite  length  of  time  without  injury. 

Cement  which  has  become  hard  or  lumpy  should  have  the 
lumps  screened  out  before  it  is  used.  The  portion  passing  the 
screen  may  be  used  with  confidence,  but  the  lumps  should  be 
discarded. 


CHAPTER  II 
CEMENT    SPECIFICATIONS    AND    TESTS 

For  the  Small  Purchaser.  —  As  was  stated  in  the  preceding 
chapter,  the  purchaser  of  small  quantities  of  cement  must  usu- 
ally rely  on  the  reputation  of  the  brand  used.  All  such  cement 
should  be  purchased,  however,  with  the  understanding  that  it 
will  meet  the  Standard  Specifications  for  Cement  of  the  Ameri- 
can Society  for  Testing  Materials.  These  specifications  have  now 
been  very  generally  adopted  throughout  this  country,  and  all 
reputable  manufacturers  will  guarantee  their  cement  to  meet 
them.  In  case  of  doubt  as  to  whether  a  cement  purchased  is  of 
good  quality,  samples  should  be  taken  as  described  later  in 
this  chapter,  and  sent  to  some  engineering  laboratory  equipped 
to  make  the  proper  tests.  Such  laboratories  may  be  found  in 
practically  all  large  cities,  and  in  the  state  colleges  and  universities. 

For  Larger  and  More  Important  Work.  —  All  cement  to  be 
used  for  larger  work  should  be  sampled  and  tested  according 
to  the  methods  recommended  by  the  American  Society  of  Civil 
Engineers,  and  should  be  required  to  meet  the  Standard  Speci- 
fications of  the  American  Society  for  Testing  Materials.  Both 
the  requirements  to  be  met  and  the  methods  of  testing  are  here 
given  in  full. 

STANDARD  SPECIFICATIONS   FOR    CEMENT 

(Adopted  by  the  American  Society  for  Testing  Materials,  August  16,  1909) 
General  Observations 

1.  These  remarks  have  been  prepared  with  a  view  of  pointing  out 
the  pertinent  features  of  the  various  requirements  and  the  precautions 
to  be  observed  in  the  interpretation  of  the  results  of  the  tests. 

2.  The  committee  would  suggest  that  the  acceptance  or  rejection 
under  these  specifications  be  based  on  tests  made  by  an  experienced 
person  having  the  proper  means  for  making  the  tests. 

13 


14      CONCRETE  CONSTRUCTION  FOR  RURAL  COMMUNITIES 

Specific  Gravity 

3.  Specific  gravity  is  useful  in  detecting  adulteration.  The  results 
of  tests  of  specific  gravity  are  not  necessarily  conclusive  as  an  indica- 
tion of  the  quality  of  a  cement,  but  when  in  combination  with  the 
results  of  other  tests  may  afford  valuable  indications. 

Fineness 

4.  The  sieves  should  be  kept  thoroughly  dry. 

Time  of  Setting 

5.  Great  care  should  be  exercised  to  maintain  the  test  pieces  under 
as  uniform  conditions  as  possible.  A  sudden  change  or  wide  range  of 
temperature  in  the  room  in  which  the  tests  are  made,  a  very  dry  or 
humid  atmosphere,  and  other  irregularities  vitally  affect  the  rate  of 
setting. 

Constancy  of  Volume 

6.  The  tests  for  constancy  of  volume  are  divided  into  two  classes: 
the  first  normal,  the  second  accelerated.  The  latter  should  be  regarded 
as  a  precautionary  test  only,  and  not  infallible.  So  many  conditions 
enter  into  the  making  and  interpreting  of  it  that  it  should  be  used 
with  extreme  care. 

7.  In  making  the  pats  the  greatest  care  should  be  exercised  to  avoid 
initial  strains  due  to  molding  or  to  too  rapid  drying  out  during  the 
first  twenty-four  hours.  The  pats  should  be  preserved  under  the  most 
uniform  conditions  possible,  and  rapid  changes  of  temperature  should 
be  avoided. 

8.  The  failure  to  meet  the  requirements  of  the  accelerated  tests 
need  not  be  sufficient  cause  for  rejection.  The  cement  may,  however, 
be  held  for  twenty-eight  days,  and  a  retest  made  at  the  end  of  that 
period,  using  a  new  sample.  Failure  to  meet  the  requirements  at  this 
time  should  be  considered  sufficient  cause  for  rejection,  although  in  the 
present  state  of  our  knowledge  it  cannot  be  said  that  such  failure  nec- 
essarily indicates  unsoundness,  nor  can  the  cement  be  considered  entirely 
satisfactory  simply  because  it  passes  the  tests. 

SPECIFICATIONS 

General  Conditions 

1.  All  cement  shall  be  inspected. 

2.  Cement  may  be  inspected  either  at  the  place  of  manufacture 
or  on  the  work. 


CEMENT  SPECIFICATIONS  AND   TESTS  15 

3.  In  order  to  allow  ample  time  for  inspecting  and  testing,  the 
cement  should  be  stored  in  a  suitable  weather-tight  building  having 
the  floor  properly  blocked  or  raised  from  the  ground. 

4.  The  cement  shall  be  stored  in  such  a  manner  as  to  permit  easy 
access  for  proper  inspection  and  identification  of  each  shipment. 

5.  Every  facility  shall  be  provided  by  the  contractor  and  a  period 
of  at  least  twelve  days  allowed  for  the  inspection  and  necessary 
tests. 

6.  Cement  shall  be  delivered  in  suitable  packages  with  the  brand 
and  name  of  manufacturer  plainly  marked  thereon. 

7.  A  bag  of  cement  shall  contain  94  pounds  of  cement  net.  Each 
barrel  of  Portland  cement  shall  contain  4  bags,  and  each  barrel  of  natu- 
ral cement  shall  contain  3  bags  of  the  above  net  weight. 

8.  Cement  failing  to  meet  the  seven-day  requirements  may  be  held 
awaiting  the  results  of  the  twenty-eight-day  tests  before  rejection. 

9.  All  tests  shall  be  made  in  accordance  with  the  methods  pro- 
posed by  the  Committee  on  Uniform  Tests  of  Cement  of  the  American 
Society  of  CivU  Engineers,  presented  to  the  Society  January  21,  1903, 
and  amended  January  20,  1904,  and  January  15,  1908,  with  all  subse- 
quent amendments  thereto. 

10.  The  acceptance  or  rejection  shall  be  based  on  the  following 
requirements: 

NATURAL  CEMENT 

11.  Definition.  —  This  term  shall  be  applied  to  the  finely  pulver- 
ized product  resulting  from  the  calcination  of  an  argillaceous  limestone 
at  a  temperature  only  sufficient  to  drive  off  the  carbonic  acid  gas. 

Fineness 

12.  It  shall  leave  by  weight  a  residue  of  not  more  than  10  per  cent 
on  the  No.  100,  and  30  per  cent  on  the  No.  200  sieve. 

Time  of  Setting 

13.  It  shall  not  develop  initial  set  in  less  than  ten  minutes;  and  shall 
not  develop  hard  set  in  less  than  thirty  minutes,  or  in  more  than  three 
hours. 

Tensile  Strength 

14.  The  minimum  requirements  for  tensile  strength  for  briquettes 
one  square  inch  in  cross  section  shall  be  as  follows,  and  the  cement  shall 
show  no  retrogression  in  strength  within  the  periods  specified: 


16     CONCRETE  CONSTRUCTION  FOR  RURAL   COMMUNITIES 

Age  Neat  Cement l  Strength 

24  hours  in  moist  air 75  lb. 

7  days  (1  day  in  moist  air,  6  days  in  water) 150  " 

28  days  (1  day  in  moist  air,  27  days  in  water)   250  " 

One  Part  Cement,  Three  Parts  Standard  Ottawa  Sand  2 

7  days  (1  day  in  moist  air,  6  days  in  water)   50  lb. 

28  days  (1  day  in  moist  air,  27  days  in  water)   125  " 

Constancy  of  Volume 

15.  Pats  of  neat  cement  about  three  inches  in  diameter,  one-half 
inch  thick  at  center,  tapering  to  a  thin  edge,  shall  be  kept  in  moist 
air  for  a  period  of  twenty-four  hours. 

(a)  A  pat  is  then  kept  in  air  at  normal  temperature. 

(b)  Another  is  kept  in  water  maintained  as  near  70°  F.  as  practicable. 

16.  These  pats  are  observed  at  intervals  for  at  least  28  days,  and, 
to  satisfactorily  pass  the  tests,  shall  remain  firm  and  hard  and  show 
no  signs  of  distortion,  checking,  cracking,  or  disintegrating. 

PORTLAND  CEMENT 

17.  Definition.  —  This  term  is  applied  to  the  finely  pulverized  prod- 
uct resulting  from  the  calcination  to  incipient  fusion  of  an  intimate 
mixture  of  properly  proportioned  argillaceous  and  calcareous  materials, 
and  to  which  no  addition  greater  than  3  per  cent  has  been  made  sub- 
sequent to  calcination. 

Specific  Gravity 

18.  The  specific  gravity  of  cement  shall  not  be  less  than  3.10. 
Should  the  test  of  cement  as  received  fall  below  this  requirement,  a 
second  test  may  be  made  upon  a  sample  ignited  at  a  low  red  heat. 
The  loss  in  weight  of  the  ignited  cement  shall  not  exceed  4  per  cent. 

Fineness 

19.  It  shall  leave  by  weight  a  residue  of  not  more  than  8  per  cent 
on  the  No.  100,  and  not  more  than  25  per  cent  on  the  No.  200  sieve. 

Time  of  Setting 

20.  It  shall  not  develop  initial  set  in  less  than  thirty  minutes; 
and  must  develop  hard  set  in  not  less  than  one  hour,  nor  more  than 
ten  hours. 

1  Neat  cement  is  cement  not  mixed  with  sand  and  stone.  —  Author. 

2  This  is  a  natural  sand  obtained  from  Ottawa,  111.,  screened  through  a 
sieve  with  20  meshes  per  linear  inch,  and  retained  on  a  sieve  with  30  meshes 
per  linear  inch.  —  Author. 


CEMENT  SPECIFICATIONS  AND   TESTS  17 

Tensile  Strength 

21.  The  minimum  requirements  for  tensile  strength  for  briquettes 
one  square  inch  in  cross  section  shall  be  as  follows,  and  the  cement 
shall  show  no  retrogression  in  strength  within  the  periods  specified: 

Age  Neat  Cement  Strength 

24  hours  in  moist  air 175  lb. 

7  days  (1  day  in  moist  air,  6  days  in  water)     500  " 

28  days  (1  day  in  moist  air,  27  days  in  water)     600  " 

One  Part  Cement,  Three  Parts  Standard  Ottawa  Sand 

7  days  (1  day  in  moist  air,  6  days  in  water)   200  lb. 

28  days  (1  day  in  moist  air,  27  days  in  water)     275  " 

Constancy  of  Volume 

22.  Pats  of  neat  cement  about  three  inches  in  diameter,  one-half 
inch  thick  at  the  center,  and  tapering  to  a  thin  edge,  shall  be  kept  in 
moist  air  for  a  period  of  twenty-four  hours. 

(a)  A  pat  is  then  kept  in  air  at  normal  temperature  and  observed 
at  intervals  for  at  least  28  days. 

(b)  Another  pat  is  kept  in  water  maintained  as  near  70°  F.  as  prac- 
ticable, and  observed  at  intervals  for  at  least  28  days. 

(c)  A  third  pat  is  exposed  in  any  convenient  way  in  an  atmosphere 
of  steam,  above  boiling  water,  in  a  loosely  closed  vessel  for  five  hours. 

23.  These  pats,  to  satisfactorily  pass  the  requirements,  shall  remain 
firm  and  hard,  and  show  no  signs  of  distortion,  checking,  cracking, 
or  disintegrating. 

Sulphuric  Acid  and  Magnesia 

24.  The  cement  shall  not  contain  more  than  1.75  per  cent  of  anhy- 
drous sulphuric  acid  (S03)  nor  more  than  4  per  cent  of  magnesia  (MgO). 

METHODS    FOR    TESTING    CEMENT 

(Recommended  by  Special  Committee  on  Uniform  Tests  of  Cement  of  the 
American  Society  of  Civil  Engineers,  January  17,  1912) 

Sampling 

1.  Selection  of  Sample.  —  The  selection  of  samples  for  testing  should 
be  left  to  the  engineer.  The  number  of  packages  sampled  and  the 
quantity  taken  from  each  package  will  depend  on  the  importance  of 
the  work  and  the  facilities  for  making  the  tests. 

2.  The  samples  should  fairly  represent  the  material.  When  the 
amount  to  be  tested  is  small  it  is  recommended  that  one  barrel  in  ten 
be  sampled;  when  the  amount  is  large  it  may  be  impracticable  to  take 
samples  from  more  than  one  barrel  in  thirty  or  fifty.    When  the  samples 


18    CONCRETE  CONSTRUCTION  FOR  RURAL  COMMUNITIES 

are  taken  from  bins  at  the  mill  one  for  each  fifty  to  two  hundred 
barrels  will  suffice. 

3.  Samples  should  be  passed  through  a  sieve  having  twenty  meshes 
per  linear  inch,  in  order  to  break  up  lumps  and  remove  foreign  ma- 
terial; the  use  of  this  sieve  is  also  effective  to  obtain  a  thorough 
mixing  of  the  samples  when  this  is  desired.  To  determine  the  accept- 
ance or  rejection  of  cement  it  is  preferable,  when  time  permits,  to  test  the 
samples  separately..  Tests  to  determine  the  general  characteristics  of  a 
cement,  extending  over  a  long  period,  may  be  made  with  mixed  samples. 

4.  Method  of  Sampling.  —  Cement  in  barrels  should  be  sampled 
through  a  hole  made  in  the  head,  or  in  one  of  the  staves  midway 
between  the  heads,  by  means  of  an  auger  or  a  sampling  iron  similar 
to  that  used  by  sugar  inspectors;  if  in  bags,  the  sample  should  be  taken 
from  surface  to  center;  cement  in  bins  should  be  sampled  in  such  a 
manner  as  to  represent  fairly  the  contents  of  the  bin.  Sampling  from 
bins  is  not  recommended  if  the  method  of  manufacture  is  such  that 
ingredients  of  any  kind  are  added  to  the  cement  subsequently. 

Chemical  Analysis 

5.  Significance.  —  Chemical  analysis  may  serve  to  detect  adultera- 
tion of  cement  with  inert  material,  such  as  slag  or  ground  limestone, 
if  in  considerable  amount.  It  is  useful  in  determining  whether  certain 
constituents,  such  as  magnesia  and  sulphuric  anhydride,  are  present  in 
inadmissible  proportions. 

6.  The  determination  of  the  principal  constituents  of  cement, 
silica,  alumina,  iron  oxide,  and  lime  is  not  conclusive  as  an  indication 
of  quality.  Faulty  cement  results  more  frequently  from  imperfect 
preparation  of  the  raw  material  or  defective  burning  than  from  in- 
correct proportions.  Cement  made  from  material  ground  very  finely 
and  thoroughly  burned  may  contain  much  more  lime  than  the  amount 
usually  present,  and  still  be  perfectly  sound.  On  the  other  hand, 
cements  low  in  lime  may,  on  account  of  careless  preparation  of  the  raw  ma- 
terial, be  of  dangerous  character.  Furthermore,  the  composition  of  the 
product  may  be  so  greatly  modified  by  the  ash  of  the  fuel  used  in  burning 
as  to  affect  in  a  great  degree  the. significances  of  the  results  of  analysis. 

7.  Methods.  —  The  methods  to  be  followed,  except  for  determining 
the  loss  on  ignition,  should  be  those  proposed  by  the  Committee  on 
Uniformity  in  the  Analysis  of  Materials  for  the  Portland  Cement 
Industry,  reported  in  the  Journal  of  the  Society  for  Chemical  Industry, 
Vol.  21,  page  12,  1902;  and  published  in  Engineering  News,  Vol.  50, 
page  60,  1903;  and  in  Engineering  Record,  Vol.  48,  page  49,  1903, 
and  in  addition  thereto,  the  following: 


CEMENT  SPECIFICATIONS  AND   TESTS  19 

(a)  The  insoluble  residue  may  be  determined  as  follows:  To  a  1-g. 
sample  of  the  cement  are  added  30  cc.  of  water  and  10  cc.  of  concen- 
trated hydrochloric  acid,  and  then  warmed  until  effervescence  ceases, 
and  digested  on  a  steam  bath  until  dissolved.  The  residue  is  filtered, 
washed  with  hot  water,  and  the  filter  paper  and  contents  digested  on 
the  steam  bath  in  a  5  per  cent  solution  of  sodium  carbonate.  This 
residue  is  filtered,  washed  with  hot  water,  then  with  hot  hydrochloric 
acid,  and  finally  with  hot  water,  and  then  ignited  at  a  red  heat  and 
weighed.     The  quantity  so  obtained  is  the  insoluble  residue. 

(b)  The  loss  on  ignition  shall  be  determined  in  the  following  manner: 
one-half  gram  of  cement  is  heated  in  a  weighed  platinum  crucible,  with 
cover,  for  5  min.  with  a  Bunsen  burner  (starting  with  a  low  flame  and  gradu- 
ally increasing  to  its  full-height)  and  then  heated  for  15  min.  with  a  blast 
lamp;  the  difference  between  the  weight  after  cooling  and  the  original 
weight  is  the  loss  on  ignition.  The  temperature  should  not  exceed  900°  C, 
or  a  low  red  heat;  the  ignition  should  preferably  be  made  in  a  muffle. 
Specific  Gravity 

8.  Significance.  —  The  specific  gravity  of  cement  is  lowered  by 
adulteration  and  hydration,  but  the  adulteration  must  be  considerable 
to  be  detected  by  tests  of  specific  gravity. 

9.  Inasmuch  as  the  differences  in  specific  gravity  are  usually  very 
small,  great  care  must  be  exercised  in  making  the  determination. 

10.  Apparatus.  —  The  determination  of  specific  gravity  should  be 
made  with  a  standardized  Le  Chatelier  apparatus.  This  consists  of 
a  flask  (D),  Fig.  2,  of  about  one  hundred  and  twenty  cubic  centi- 
meters capacity,  the  neck  of  which  is  about  twenty  centimeters  long; 
in  the  middle  of  this  neck  is  a  bulb  (C),  above  and  below  which  are 
two  marks  (F)  and  (E) ;  the  volume  between  these  two  marks  is  20  cc. 
The  neck  has  a  diameter  of  about  nine  millimeters,  and  is  graduated 
into  tenths  of  cubic  centimeters  above  the  mark  (F). 

11.  Benzine  (62°  Baume*  naphtha)  or  kerosene  free  from  water 
should  be  used  in  making  the  determination. 

12.  Method.  —  The  flask  is  filled  with  either  of  these  liquids  to  the 
lower  mark  (E),  and  64  g.  of  cement,  cooled  to  the  temperature  of  the 
liquid,  is  slowly  introduced  through  the  funnel  (B),  —  the  stem  of  which 
should  be  long  enough  to  extend  into  the  flask  to  the  top  of  the  bulb 
(C), — taking  care  that  the  cement  does  not  adhere  to  the  sides  of  the 
flask,  and  that  the  funnel  does  not  touch  the  liquid.  After  all  the  ce- 
ment is  introduced,  the  level  of  the  liquid  will  rise  to  some  division  of 
the  graduated  neck;  this  reading,  plus  20  cc,  is  the  volume  displaced 
by  64  g.  of  the  cement. 


20    CONCRETE  CONSTRUCTION  FOR  RURAL   COMMUNITIES 
13.   The  specific  gravity  is  then  obtained  from  the  formula,  — 


Specific  gravity 


Weight  of  cement  in  grams 


Displaced  volume  in  cubic  centimeters 
14.   The  flask,  during  the  operation,  is  kept  immersed  in  water  in  a 
jar  (A),  in  order  to  avoid  variations  in  the  temperature  of  the  liquid 

B 


Fig.  2.  —  Le  Chatelier's  Specific  Gravity  Apparatus. 

in  the  flask,  which  should  not  exceed  \°  C.  The  results  of  repeated 
tests  should  agree  within  0.01.  The  determination  of  specific  gravity 
should  be  made  on  the  cement  as  received;  if  it  should  fall  below 
3.10,  a  second  determination  should  be  made  after  igniting  the  sample 
in  a  covered  dish,  preferably  of  platinum,  at  a  low  red  heat  not  exceed- 
ing 900°  C.  The  sample  should  be  heated  for  5  min.  with  a  Bunsen 
burner  (starting  with  a  low  flame  and  gradually  increasing  to  its  full 
height)  and  then  heated  for  15  min.  with  a  blast  lamp;  the  ignition 
should  preferably  be  made  in  a  muffle. 

15.  The  apparatus  may  be  cleaned  in  the  following  manner:  The 
flask  is  inverted  and  shaken  vertically  until  the  liquid  flows  freely,  and 
then  held  in  a  vertical  position  until  empty;  any  traces  of  cement 
remaining  can  be  removed  by  pouring  into  the  flask  a  small  quantity 
of  clean  liquid  benzine  or  kerosene  and  repeating  the  operation. 


CEMENT  SPECIFICATIONS  AND   TESTS 


21 


Fineness 

16.  Significance.  —  It  is  generally  accepted  that  the  coarser  parti- 
cles in  cement  are  practically  inert,  and  it  is  only  the  extremely  fine 
powder  that  possesses  cementing  qualities.  The  more  finely  cement  is 
pulverized,  other  conditions  being  the  same,  the  more  sand  it  will  carry 
and  produce  a  mortar  of  a  given  strength. 

17.  Apparatus.  —  The  fineness  of  a  sample  of  cement  is  determined 
by  weighing  the  residue  retained  on  certain  sieves.  Those  known  as 
No.  100  and  No.  200,  having  approximately  100  and  200  wires  per 
linear  inch,  respectively,  should  be  used.  They  should  be  8  in.  in  diam- 
eter. The  frame  should  be  of  brass,  and  the  sieve  of  brass  wire  cloth 
conforming  to  the  following  requirements: 


No.  of  sieve 

Diameter  of  wire, 
inches 

Meshes  per  linear  inch 

Warp 

Woof 

100 
200 

0.0042  to  0.0048 
0.0021  to  0.0023 

95  to  101 
192  to  203 

93  to  103 
190  to  205 

The  meshes  in  any  smaller  space,  down  to  0.25  in.,  should  be  pro- 
portional in  number. 

18.  Method.  —  The  test  should  be  made  with  50  g.  of  cement,  dried 
at  a  temperature  of  100°  C.  (212°  F.). 

19.  The  cement  is  placed  on  the  No.  200  sieve,  which,  with  pan 
and  cover  attached,  is  held  in  one  hand  in  a  slightly  inclined  position, 
and  moved  forward  and  backward  about  200  times  per  minute,  at 
the  same  time  striking  the  side  gently,  on  the  up  stroke,  against  the 
palm  of  the  other  hand.  The  operation  is  continued  until  not  more 
than  0.05  g.  will  pass  through  in  1  min.  The  residue  is  weighed,  then 
placed  on  the  No.  100  sieve,  and  the  operation  repeated.  The  work 
may  be  expedited  by  placing  in  the  sieve  a  few  large  steel  shot,  which 
should  be  removed  before  the  final  1  min.  of  sieving.  The  sieves 
should  be  thoroughly  dry  and  clean. 

Normal  Consistency 

20.  Significance.  —  The  use  of  a  proper  percentage  of  water  in 
making  pastes1  and  mortars  for  the  various  tests  is  exceedingly  impor- 
tant and  affects  vitally  the  results  obtained. 

1  The  term  "paste"  is  used  in  this  report  to  designate  a  mixture  of 
cement  and  water,  and  the  word  "mortar"  to  designate  a  mixture  of 
cement,  sand,  and  water. 


22    CONCRETE  CONSTRUCTION   FOR  RURAL   COMMUNITIES 

21.  The  amount  of  water,  expressed  in  percentage  by  weight  of  the 
dry  cement,  required  to  produce  a  paste  of  plasticity  desired,  termed 
"normal  consistency, "  should  be  determined  with  the  Vicat  apparatus 
in  the  following  manner: 


|B 


I 


Fig.   3.  —  Vicat  Apparatus. 

22.  Apparatus.  —  This  consists  of  a  frame  (A)  Fig.  3,  bearing  a 
movable  rod  (B),  weighing  300  g.,  one  end  (C)  being  1  cm.  in  diam- 
eter for  a  distance  of  6  cm.,  the  other  having  a  removable  needle  (D), 
1  mm.  in  diameter,  6  mm.  long.  The  rod  is  reversible,  and  can  be 
held  in  any  desired  position  by  a  screw  (E),  and  has  midway  between 
the  ends  a  mark  (F)  which  moves  under  a  scale  (graduated  to  millimeters) 
attached  to  the  frame  (A).  The  paste  is  held  in  a  conical,  hard- 
rubber  ring  (G),  7  cm.  in  diameter  at  the  base,  4  cm.  high,  resting  on  a 
glass  plate  (H)  about  10  cm.  square. 

23.  Method.  —  In  making  the  determination,  the  same  quantity  of 
cement  as  will  be  used  subsequently  for  each  batch  in  making  the 
test  pieces,  but  not  less  than  500  g.,  with  a  measured  quantity  of 
water,  is  kneaded  into  a  paste,  as  described  in  Paragraph  45,  and 
quickly  formed  into  a  ball  with  the  hands,  completing  the  operation 
by  tossing  it  six  times  from  one  hand  to  the  other,  maintained  about 


CEMENT  SPECIFICATIONS  AND   TESTS 


23 


6  in.  apart;  the  ball  resting  in  the  palm  of  one  hand  is  pressed  into 
the  larger  end  of  the  rubber  ring  held  in  the  other  hand,  completely 
filling  the  ring  with  paste;  the  excess  at  the  larger  end  is  then  re- 
moved by  a  single  movement  of  the  palm  of  the  hand;  the  ring  is 
then  placed  on  its  larger  end  on  a  glass  plate  and  the  excess  paste  at 
the  smaller  end  is  sliced  off  at  the  top  of  the  ring  by  a  single  oblique 
stroke  of  a  trowel  held  at  a  slight  angle  with  the  top  of  the  ring.  Dur- 
ing these  operations  care  must  be  taken  not  to  compress  the  paste. 
The  paste  confined  in  the  ring,  resting  on  the  plate,  is  placed  under 
the  rod,  the  larger  end  of  which  is  brought  in  contact  with  the  surface 
of  the  paste;   the  scale  is  then  read,  and  the  rod  quickly  released. 

24.  The  paste  is  of  normal  consistency  when  the  cylinder  settles  to 
a  point  10  mm.  below  the  original  surface  in  one-half  minute  after  being 
released.  The  apparatus  must  be  free  from  all  vibrations  during  the 
test. 

25.  Trial  pastes  are  made  with  varying  percentages  of  water  until 
the  normal  consistency  is  obtained. 

26.  Having  determined  the  percentage  of  water  required  to  produce 
a  paste  of  normal  consistency,  the  percentage  required  for  a  mortar 
containing,  by  weight,  one  part  of  cement  to  three  parts  of  standard 
Ottawa  sand,  is  obtained  from  the  following  table,  the  amount  being 
a  percentage  of  the  combined  weight  of  the  cement  and  sand. 

Table  II 
Percentage  of  Water  for  Standard  Mortars 


One  cement, 

One  cement, 

One  cement, 

Neat 

three  standard 

Neat 

three  standard 

Neat 

three  standard 

Ottawa  sand 

Ottawa  sand 

Ottawa  sand 

15 

8.0 

23 

9.3 

31 

10.7 

16 

8.2 

24 

9.5 

32 

10.8 

17 

8.3 

25 

9.7 

33 

11.0 

18 

8.5 

26 

9.8 

34 

11.2 

19 

8.7 

27    • 

10.0 

35 

11.3 

20 

8.8 

28 

10.2 

36 

11.5 

21 

9.0 

29 

10.3 

37 

11.7 

22 

9.2 

30 

10.5 

38 

11.8 

Time  of  Setting 

27.  Significance.  —  The  object  of  this  test  is  to  determine  the  time 
which  elapses  from  the  moment  water  is  added  until  the  paste  ceases 
to  be  plastic  (called  the  "initial  set"),  and  also  the  time  until  it  ac- 


24    CONCRETE  CONSTRUCTION  FOR  RURAL  COMMUNITIES 

quires  a  certain  degree  of  hardness  (called  the  "final  set"  or  "hard 
set").  The  former  is  the  more  important,  since,  with  the  commence- 
ment of  setting,  the  process  of  crystallization  begins.  As  a  disturbance 
of  this  process  may  produce  a  loss  of  strength,  it  is  desirable  to  complete 
the  operation  of  mixing  or  molding  or  incorporating  the  mortar  into 
the  work  before  the  cement  begins  to  set. 

28.  Apparatus.  —  The  initial  and  final  set  should  be  determined 
with  the  Vicat  apparatus  described  in  Paragraph  22. 

29.  Method.  —  A  paste  of  normal  consistency  is  molded  in  the  hard- 
rubber  ring,  as  described  in  Paragraph  23,  and  placed  under  the  rod 
(B),  the  smaller  end  of  which  is  then  carefully  brought  in  contact 
with  the  surface  of  the  paste,  and  the  rod  quickly  released. 

30.  The  initial  set  is  said  to  have  occurred  when  the  needle  ceases 
to  pass  a  point  5  mm.  above  the  glass  plate;  and  the  final  set,  when 
the  needle  does  not  sink  visibly  into  the  paste. 

31.  The  test  pieces  should  be  kept  in  moist  air  during  the  test; 
this  may  be  accomplished  by  placing  them  on  a  rack  over  water  con- 
tained in  a  pan  and  covered  by  a  damp  cloth;  the  cloth  to  be  kept 
from  contact  with  them  by  means  of  a  wire  screen;  or  they  may  be 
stored  in  a  moist  box  or  closet. 

32.  Care  should  be  taken  to  keep  the  needle  clean,  as  the  collec- 
tion of  cement  on  the  sides  of  the  needle  retards  the  penetration,  while 
cement  on  the  point  may  increase  the  penetration. 

33.  The  time  of  setting  is  affected  not  only  by  the  percentage  and 
temperature  of  the  water  used  and  the  amount  of  kneading  the  paste 
receives,  but  by  the  temperature  and  humidity  of  the  air,  and  its 
determination  is,  therefore,  only  approximate. 

Standard  Sand 

34.  The  sand  to  be  used  should  be  natural  sand  from  Ottawa,  111.,1 
screened  to  pass  a  No.  20  sieve,  and  retained  on  a  No.  30  sieve.  The 
sieves  should  be  at  least  8  in.  in  diameter;  the  wire  cloth  should  be  of 
brass  wire  and  should  conform  to  the  following  requirements: 


No.  of  sieve 

Diameter  of  wire, 
inches 

Meshes  per  linear  inch 

Warp 

Woof 

20 
30 

0.016  to  0.017 
0.011  to  0.012 

19.5  to  20.5 
29.5  to  30.5 

19     to  21 
28.5  to  31.5 

This  may  be  obtained  from  the  Ottawa  Silica  Co.,  Ottawa,  111. 


CEMENT  SPECIFICATIONS  AND   TESTS 


25 


Sand  which  has  passed  the  No.  20  sieve  is  standard  when  not  more 
than  5  g.  passes  the  No.  30  sieve  in  1  min.  of  continuous  sifting  of  a 
500-gram  sample. 

Form  of  Test  Pieces 

35.  For  tensile  tests,  the  form  of  test  piece  shown  in  Fig.  4  should 
be  used. 


i 


Fig.  4.  —  Details  for  Briquette. 

36.  For  compressive  tests,  2-in.  cubes  should  be  used. 

Molds 

37.  The  molds  should  be  of  brass,  bronze,  or  other  non-corrodible 
material,  and  should  have  sufficient  metal  in  the  sides  to  prevent 
spreading  during  molding. 


26     CONCRETE  CONSTRUCTION  FOR  RURAL   COMMUNITIES 

38.  Molds  may  be  either  single  or  gang  molds.  The  latter  are  pre- 
ferred by  many.  If  used,  the  types  shown  in  Figs.  5  and  6  are  recom- 
mended. 


I 


Fig.  5.  —  Details  for  Gang  Mold. 


Fig.  6.  —  Mold  for  Compression  Test  Pieces. 

39.  The  molds  should  be  wiped  with  an  oily  cloth  before  using. 

Mixing 

40.  The  proportions  of  sand  and  cement  should  be  stated  by  weight; 
the  quantity  of  water  should  be  stated  as  a  percentage  by  weight  of 
the  dry  material. 

41.  The  metric  system  is  recommended  because  of  the  convenient 
relation  of  the  gram  and  the  cubic  centimeter. 

42.  The  temperature  of  the  room  and  of  the  mixing  water  should 
be  maintained  as  nearly  as  practicable  at  21°  C.  (70°  F.). 

43.  The  quantity  of  material  to  be  mixed  at  one  time  depends  on 
the  number  of  test  pieces  to  be  made;  1000  grams  is  a  convenient 
quantity  to  mix  by  hand  methods. 

44.  The  Committee  has  investigated  the  various  mechanical  mixing 
machines  thus  far  devised,  but  cannot  recommend  any  of  them,  for 
the  following  reasons:  (1)  The  tendency  of  most  cement  is  to  "ball 
up  "  in  the  machine,  thereby  preventing  working  it  into  a  homogeneous 
paste;  (2)  there  are  no  means  of  ascertaining  when  the  mixing  is  com- 
plete without  stopping  the  machine;  and  (3)  it  is  difficult  to  keep  the 
machine  clean. 

45.  Method.  —  The  material  is  weighed,  placed  on  a  non-absorbent 
surface  (preferably  plate  glass),  thoroughly  mixed  dry  if  sand  be  used, 
and  a  crater  formed  in  the  center,  into  which  the  proper  percentage 
of  clean  water  is  poured;  the  material  on  the  outer  edge  is  turned  into 
the  center  by  the  aid  of  a  trowel.     As  soon  as  the  water  has  been 


CEMENT  SPECIFICATIONS  AND   TESTS  27 

absorbed,  which  should  not  require  more  than  one  minute,  the  opera- 
tion is  completed  by  vigorously  kneading  with  the  hands  for  1  min. 
During  the  operation  the  hands  should  be  protected  by  rubber  gloves. 

Molding 

46.  The  Committee  has  not  been  able  to  secure  satisfactory  results 
with  existing  molding  machines;  the  operation  of  machine  molding  is 
very  slow;  and  is  not  practicable  with  pastes  or  mortars  containing 
as  large  percentages  of  water  as  herein  recommended. 

47.  Method. —  Immediately  after  mixing,  the  paste  or  mortar  is 
placed  in  the  molds  with  the  hands,  pressed  in  firmly  with  the  fingers, 
and  smoothed  off  with  a  trowel  without  ramming.  The  material  should 
be  heaped  above  the  mold,  and,  in  smoothing  off,  the  trowel  should  be 
drawn  over  the  mold  in  such  a  manner  as  to  exert  a  moderate  pressure 
on  the  material.  The  mold  should  then  be  turned  over  and  the  opera- 
tion of  heaping  and  smoothing  off  repeated. 

48.  A  check  on  the  uniformity  of  mixing  and  molding  may  be 
afforded  by  weighing  the  test  pieces  on  removal  from  the  moist  closet; 
test  pieces  from  any  sample  which  vary  in  weight  more  than  3  per 
cent  from  the  average  should  be  not  considered. 

Storage  of  the  Test  Pieces 

49.  During  the  first  24  hours  after  molding,  the  test  pieces  should 
be  kept  in  moist  air  to  prevent  drying. 

50.  Two  methods  are  in  common  use  to  prevent  drying:  (1)  cover- 
ing the  test  pieces  with  a  damp  cloth,  and  (2)  placing  them  in  a  moist 
closet.  The  use  of  the  damp  cloth,  as  usually  carried  out,  is  objec- 
tionable, because  the  cloth  may  dry  out  unequally  and  in  consequence 
the  test  pieces  will  not  all  be  subjected  to  the  same  degree  of  moisture. 
This  defect  may  be  remedied  to  some  extent  by  immersing  the  edges 
of  the  cloth  in  water;  contact  between  the  cloth  and  the  test  pieces 
should  be  prevented  by  means  of  a  wire  screen,  or  some  similar  arrange- 
ment. A  moist  closet  is  so  much  more  effective  in  securing  uniformly 
moist  air,  and  is  so  easily  devised  and  so  inexpensive,  that  the  use  of 
the  damp  cloth  should  be  abandoned. 

51.  A  moist  closet  consists  of  a  soapstone  or  slate  box,  or  a  wooden 
box  lined  with  metal,  the  interior  surface  being  covered  with  felt  or 
broad  wicking  kept  wet,  the  bottom  of  the  box  being  kept  covered 
with  water.  The  interior  of  the  box  is  provided  with  glass  shelves  on 
which  to  place  the  test  pieces,  the  shelves  being  so  arranged  that  they 
may  be  withdrawn  readily. 

52.  After  24    hours    in    moist  air,  the    pieces  to   be    tested    after 


28    CONCRETE  CONSTRUCTION  FOR   RURAL  COMMUNITIES 

longer  periods  should  be  immersed  in  water  in  storage  tanks  or  pans 
made  of  non-corrodible  material. 

53.  The  air  and  water  in  the  moist  closet  and  the  water  in  the 
storage  tanks  should  be  maintained  as  nearly  as  practicable  at  21°  C. 
(70°  F.). 

Tensile  Strength 

54.  The  tests  may  be  made  with  any  standard  machine. 

55.  The  clip  is  shown  in  Fig.  7.    It  must  be  made  accurately,  the 


Roller  Turned  and  Accurately 
Bored  to  easy  Turning  Fit 


Fig.  7.  —  Form  of  Clip. 


pins  and  rollers  turned,  and  the  rollers  bored  slightly  larger  than  the 
pins  so  as  to  turn  easily.  There  should  be  a  slight  clearance  at  each 
end  of  the  roller,  and  the  pins  should  be  kept  properly  lubricated  and 
free  from  grit.  The  clips  should  be  used  without  cushioning  at  the 
points  of  contact. 

56.  Test  pieces  should  be  broken  as  soon  as  they  are  removed  from 


CEMENT  SPECIFICATIONS  AND   TESTS 


29 


the  water.  Care  should  be  observed  in  centering  the  test  pieces  in  the 
testing  machine,  as  cross  strains,  produced  by  imperfect  centering, 
tend  to  lower  the  breaking  strength.  The  load  should  not  be  applied 
too  suddenly,  as  it  may  produce  vibration,  the  shock  from  which  often 
causes  the  test  pieces  to  break  before  the  ultimate  strength  is  reached. 
The  bearing  surfaces  of  the  clips  and  test  pieces  must  be  kept  free  from 
grains  of  sand  or  dirt,  which  would  prevent  a  good  bearing.  The 
load  should  be  applied  at  the  rate  of  600  lbs.  per  min.    The  average 


Fig.  8.  —  Ball-bearing  Block  for  Testing  Machine. 


of  the  results  of  the  test  pieces  from  each  sample  should  be  taken  as 
the  test  of  the  sample.  Test  pieces  which  do  not  break  within  I  in. 
of  the  center,  or  are  otherwise  manifestly  faulty,  should  be  excluded 
in  determining  average  results. 

Compressive  Strength 

57.  The  tests  may  be  made  with  any  machine  provided  with  means 
for  so  applying  the  load  that  the  line  of  pressure  is  along  the  axis 
of  the  test  piece.  A  ball-bearing  block  for  this  purpose  is  shown  in 
Fig.  8.  Some  appliance  should  be  provided  to  facilitate  placing  the  axis 
of  the  test  piece  exactly  in  line  with  the  center  of  the  ball-bearing. 


30    CONCRETE  CONSTRUCTION  FOR  RURAL  COMMUNITIES 

58.  The  test  piece  should  be  placed  in  the  testing  machine,  with  a 
piece  of  heavy  blotting  paper  on  each  of  the  crushing  faces,  which 
should  be  those  that  were  in  contact  with  the  mold. 

Constancy  of  Volume 

59.  Significance.  —  The  object  is  to  detect  those  qualities  which 
tend  to  destroy  the  strength  and  durability  of  a  cement.  Under  nor- 
mal conditions  these  defects  will  in  some  cases  develop  quickly,  and 
in  other  cases  may  not  develop  for  a  considerable  time.  Since  the 
detection  of  these  destructive  qualities  before  using  the  cement  in  con- 
struction is  essential,  tests  are  made  not  only  under  normal  conditions 
but  under  artificial  conditions  created  to  hasten  the  development  of 
these  defects.  Tests  may,  therefore,  be  divided  into  two  classes:  (1) 
Normal  tests,  made  in  either  air  or  water  maintained,  as  nearly  as 
practicable,  at  21°  C.  (70°  F.);  and  (2)  Accelerated  tests,  made  in  air, 
steam  or  water,  at  temperature  of  45°  C.  (113°  F.)  and  upward.  The 
Committee  recommends  that  these  tests  be  made  in  the  following 
manner: 

60.  Methods.  —  Pats,  about  three  inches  in  diameter,  one-half  inch 
thick  at  the  center,  and  tapering  to  a  thin  edge,  should  be  made  on 
clean  glass  plates  (about  four  inches  square)  from  cement  paste  of 
normal  consistency,  and  stored  in  a  moist  closet  for  24  hours. 

61.  Normal  Tests.  —  After  24  hours  in  the  moist  closet,  a  pat  is 
immersed  in  water  for  28  days  and  observed  at  intervals.  A  similar 
pat,  after  24  hours  in  the  moist  closet,  is  "exposed  to  the  air  for  28 
days  or  more  and  observed  at  intervals. 

62.  Accelerated  Test.  —  After  24  hours  in  the  moist  closet,  a  pat  is 
placed  in  an  atmosphere  of  steam,  upon  a  wire  screen  1  in.  above 
boiling  water  for  5  hours.  The  apparatus  should  be  so  constructed 
that  the  steam  will  escape  freely  and  atmospheric  pressure  be  main- 
tained. Since  the  type  of  apparatus  used  has  a  great  influence  on  the 
results,  the  arrangement  shown  in  Fig.  9  is  recommended. 

63.  Pats  which  remain  firm  and  hard  and  show  no  signs  of  crack- 
ing, distortion,  or  disintegration  are  said  to  be  "of  constant  volume" 
or  "sound." 

64.  Should  the  pat  leave  the  plate,  distortion  may  be  detected 
best  with  a  straight-edge  applied  to  the  surface  which  was  in  contact 
with  the  plate. 

65.  In  the  present  state  of  our  knowledge  it  cannot  be  said  that  a 
cement  which  fails  to  pass  the  accelerated  test  will  prove  defective  in 
the  work;  nor  can  a  cement  be  considered  entirely  safe  simply  because 
it  has  passed  these  tests. 


CEMENT  SPECIFICATIONS  AND   TESTS 


31 


aqnxjaqqnaaiqtxau'^ 


Hoooqaaij    -h-H 


32    CONCRETE  CONSTRUCTION  FOR  RURAL  COMMUNITIES 

Soundness  Test  without  Special  Apparatus.  —  Of  the  above 
tests,  the  one  relating  to  constancy  of  volume  or  soundness  is 
probably  the  most  important.  Fortunately  this  test  can  easily 
be  made  by  anyone  who  is  careful,  without  any  special  appara- 
tus or  training.  The  cement  should  be  sampled  as  previously 
described  in  this  chapter.  This  sample  should  then  be  thoroughly 
mixed  with  water  on  some  nonabsorbent  surface,  such  as  glass 
or  porcelain,  to  the  consistency  of  putty.  What  is  known  as  the 
Boulogne  method  of  determining  the  proper  consistency  requires 
that  "the  paste  shall  be  firm  but  well  bonded,  shining  and 
plastic,  and  shall  satisfy  the  following  conditions: 

"l.  The  consistency  shall  not  change  if  it  is  worked  3  minutes 
longer  than  the  original  5  minutes. 

"2.  If  dropped  20  inches  from  a  trowel,  it  should  leave  the  trowel 
clean,  and  fall  without  losing  its  shape  or  cracking. 

"3.  Light  pressure  in  the  hand  should  bring  water  to  the  surface, 
and  the  paste  should  not  stick  to  the  hand.  If  a  ball  thus  formed 
falls  from  a  height  of  about  20  inches,  it  should  retain  its  rounded 
form  without  showing  cracks. 

"4.  The  proportion  of  water  should  be  such  that  more  or  less  will 
produce  opposite  effects  from  those  just  described  for  the  proper 
consistency."1 

After  repeated  trials  have  given  a  paste  which  is  just  right, 
it  should  be  made,  on  pieces  of  glass,  into  three  or  more  pats, 
about  3  inches  in  diameter  and  one-half  inch  thick  at  the  middle, 
tapering  to  a  thin  edge.  These  pats  should  be  kept  in  moist 
air  for  the  first  twenty-four  hours.  This  may  be  done  by  sus- 
pending a  damp  cloth,  such  as  a  piece  of  burlap,  over  the  pats 
so  that  it  does  not  touch  them  but  has  its  ends  dipping  into 
pans  of  water.  After  twenty-four  hours  one  of  the  pats  should 
be  placed  in  dry  air  and  one  in  water,  and  left  for  twenty-seven 
days,  observations  being  taken  at  intervals.  The  third  pat 
should  be  suspended  over  boiling  water  for  five  hours  in  a 
loosely  closed  vessel,  such  as  a  teakettle.  The  pat  should  be 
far  enough  above  the  water  so  that  the  latter  will  not  strike  it 
in  boiling. 

1  Taylor  and  Thompson,  "Concrete,  Plain  and  Reinforced,"  2d  edition, 
page  71. 


CEMENT  SPECIFICATIONS  AND   TESTS 


33 


Any  radial  cracks  formed  near  the  edges,  or  any  evidences  of 
crumbling  of  the  air  or  water  pats,  show  the  cement  to  be  un- 
sound, and  call  for  its  rejection.  Cracks  which  are  sometimes 
found  near  the  middle  of  the  pat  are  usually  evidence  that  the 
air  in  which  the  pats  were  kept  during  the  first  twenty-four 
hours  was  not  moist  enough,  and  these  should  not  be  confused 
with  the  evidences  of  unsoundness.     Figures  10  and  11  show 


Fig.  10.  —  Pat  showing 
Shrinkage  Cracks  due 
to  too  Rapid  Drying. 
These  Cracks  are 
Harmless. 


Fig.  11.  —  Pat  showing 
Expansion  Cracks  due 
to  Unsoundness  of  Ce- 
ment. These  warrant 
Rejection  of  the  Cement. 


the  difference  between  dangerous  and  harmless  cracks.  If  the 
pat  which  is  given  the  steaming  test  shows  radial  cracks  at  the 
edges,  the  cement  is  not  necessarily  unsound,  but  should  not  be 
used  until  the  end  of  the  twenty-eight-day  tests  of  the  air  and 
water  pats.  If  the  latter  pats  are  all  right,  the  cement  may 
be  used,  notwithstanding  the  failure  of  the  pat  under  the  accel- 
erated test.  On  the  other  hand,  if  the  pat  passes  the  steaming 
test,  the  cement  may  be  used,  without  waiting  for  the  twenty- 
eight-day  tests. 


CHAPTER  III 
AGGREGATES 

Aggregates.  —  The  name  aggregates  is  given  to  those  mate- 
rials, such  as  sand,  gravel,  broken  stone,  and  cinders,  which  are 
bound  together  by  the  cement  to  form  concrete.  These  are 
divided  into  two  classes:  (1)  fine  aggregates,  such  as  sand  and 
stone  screenings;  and  (2)  coarse  aggregates,  such  as  gravel  and 
broken  stone.  For  concrete,  it  is  customary  to  use  a  mixture 
of  coarse  and  fine  aggregates  in  about  the  proportion  of  two 
parts  of  the  former  to  one  part  of  the  latter,  though  sometimes 
the  coarse  aggregate  is  omitted  altogether,  or  a  mixture  of 
coarse  and  fine  material  may  be  used  as  it  comes  from  the 
gravel  bank.  The  subject  of  proportions  will  be  discussed 
fully  in  the  next  chapter. 

Selection  of  Aggregates.  —  The  quality  of  the  aggregates  used 
has  a  very  important  effect  on  the  quality  of  the  concrete  pro- 
duced, and  a  careful  study  of  them  will  be  well  worth  while. 
Cements  are  manufactured  under  careful  supervision,  samples 
of  the  product  being  taken  at  regular  intervals  and  carefully 
tested  by  the  manufacturers.  The  aggregates,  on  the  other  hand, 
are  usually  local  materials,  often  being  taken  from  the  site  of 
the  work,  and  not  subjected  to  expert  inspection  or  to  tests  of 
their  quality.  Consequently,  much  more  poor  concrete  results 
from  using  poor  aggregates  than  from  using  poor  cement.  Fur- 
ther, certain  aggregates  will  produce  a  concrete  of  given  quality 
with  a  much  smaller  proportion  of  cement  than  will  other  ag- 
gregates. Tests  show  that  a  very  fine  sand  may  require  several 
times  as  much  cement  as  a  coarse  sand  requires  to  produce  the 
same  strength  of  concrete.  As  cement  is  the  most  expensive 
element  in  concrete,  it  will,  therefore,  often  be  uneconomical 
to  use  the  cheapest  sand  or  gravel,  or  the  one  which  can  be 
had  with  the  least  hauling.  The  quality  of  the  aggregates  should 
be  carefully  studied,  and  only  those  should  be  used  which  will 

34 


AGGREGATES  35 

produce  a  sound  concrete  of  a  strength  sufficient  for  the  purpose, 
at  the  lowest  total  cost,  all  things  considered. 

Sand.  —  Sand  is  most  commonly  used  for  the  fine  aggregate 
in  concrete.  As  a  matter  of  convenience  the  term  may  be  re- 
stricted to  particles  which  will  pass  through  a  J-inch  mesh 
screen,  the  coarser  particles  being  considered  as  gravel.  It  may 
come  either  from  a  natural  sand-bank,  or  from  a  river,  a  lake, 
or  a  sea.  Sands  from  each  of  these  sources  are  extensively 
used  in  concrete  construction. 

Requirements  of  Sand.  —  The  principal  requirements  of  sand 
for  concrete  are  that  it  be  coarse  and  clean.  It  is  often  speci- 
fied that  the  sand  shall  be  sharp.  Careful  tests  show  that  this 
is  entirely  unnecessary,  sands  with  rounded  grains  being  in 
every  respect  as  suitable  for  the  purpose  as  those  with  angular 
grains,  usually  designated  as  sharp  sands.  The  terms  coarse 
and  clean  are  rather  indefinite,  as  sand  which  would  be  regarded 
as  coarse  in  some  parts  of  the  country  would  be  called  fine  in 
others,  and  while  a  considerable  amount  of  some  impurities  in 
sand  is  harmless,  a  minute  quantity  of  others  will  destroy  its 
usefulness  for  concrete.  It  may  be  said  in  general  that  the 
coarser  the  sand  is,  the  stronger  concrete  it  will  make,  other 
things  being  equal,  and  that  sand  containing  a  mixture  of 
coarse  and  fine  particles,  with  the  coarse  grains  predominating, 
is  ideal.  Probably  most  coarse  sands  have  at  least  enough  fine 
sand  in  them  as  they  come  from  the  bank  or  bed.  The  fine 
grains  will  be  hidden  between  the  coarser  ones,  and  will  escape 
notice,  so  that  a  sand  which,  on  superficial  examination,  appears 
to  be  made  up  almost  wholly  of  coarse  grains  will,  upon  screen- 
ing, show  a  considerable  percentage  of  fine  particles. 

Use  of  Fine  Sand.  —  The  relative  coarseness  of  sand  is  best 
determined  by  screening  a  sample  over  a  set  of  screens  of  dif- 
ferent meshes  l  and  noting  the  percentage  of  the  sample  passing 
each  screen.  As  an  approximate  guide  to  the  meaning  usually 
given  to  the  terms  coarse,  medium,  and  fine,  particles  which 
would  be  retained  on  a  15-mesh  screen  may  be  considered  as 
coarse,  those  passing  a  15-mesh  screen   and   retained  on  a  40- 

1  By  the  mesh  of  a  screen  is  meant  the  number  of  meshes  or  openings  in 
one  linear  inch,  measured  at  right  angles  to  the  wires.  Ordinary  window 
screen  is  about  twelve  mesh. 


36     CONCRETE   CONSTRUCTION  FOR   RURAL   COMMUNITIES 

mesh  screen  as  medium,  and  those  passing  a  40-mesh  screen  as 
fine. 

A  sand  containing  a  large  percentage  of  fine  particles  will 
usually  give  a  concrete  of  low  strength,  and  one  which  is  readily 
penetrated  by  water,  unless  a  large  percentage  of  cement  is 
used.  One  reason  for  this  is  that  when  the  particles  are  fine, 
the  total  surface  area  of  the  particles  in  a  given  volume  is  con- 
siderably greater  than  when  the  particles  are  coarse,  and  hence 
more  cement  is  required  to  coat  them  as  perfectly.  It  is  also 
more  difficult  to  get  the  cement  paste  well  distributed  through 
the  sand  when  the  particles  of   the  latter  are  fine   than  when 

they  are  coarse. 

Table  III » 

Effect  of  Fineness  of  Sand  on  Strength  of  1 : 3  Mortar 


Sand 
No. 

Per  cent  passing  sieves  of  given  mesh 

Strength  lbs.  per  sq. 
in.     Age  28  days 

10 

20 

30 

40 

50 

80 

100 

Tensile 

Compressive 

1 
2 
3 
4 
5 
6 

73 
84 
93 
93 
92 
99 

46 
69 

78 
71 
84 
97 

30 
53 
61 
59 
76 
95 

15 
32 

38 
46 
62 

87 

7 
14 
16 
30 
38 
61 

2 

5 

4 

15 

12 

16 

1 
3 
2 
8 
5 
6 

654 
622 
527 
444 
361 
335 

5600 
4099 
3415 
3159 
3112 
1898 

Table  III  shows  the  effect  of  fineness  of  sand  on  the  strength 
of  mortar.  Sand  No.  6,  with  87  per  cent  passing  the  No.  40 
sieve,  gives  a  mortar  only  about  one-half  as  strong  in  tension, 
and  one-third  as  strong  in  compression,  as  that  from  sand  No.  1,  of 
which  only  15  per  cent  passes  the  No.  40  sieve.  The  other  sands 
also  show  a  decrease  of  strength  with  the  fineness  of  the  sand. 

If  the  sand  contains  too  large  a  percentage  of  fine  particles,  it 
will  sometimes  pay  to  screen  it  over  a  40-  or  50-mesh  screen,  and 
reject  the  particles  passing  through.  If  this  is  not  done,  a  large 
percentage  of  cement  should  be  used  to  give  the  necessary  strength. 

Uniformity  Coefficient.  —  It  is  desirable  that  the  particles  of 
a  sand  should  vary  in  size  from  fine  to  coarse,  with  the  coarser 

1  Prepared  from  tests  reported  in  Bulletin  No.  331,  United  States  Geologi- 
cal Survey. 


AGGREGATES 


37 


ones  predominating,  since  the  fine  grains  will  help  to  fill  up  the 
spaces  between  the  coarse  grains  and  so  reduce  the  amount  of 
cement  required  for  this  purpose.  The  effect  of  a  proper  grada- 
tion of  the  sand  grains  on  the  strength  of  mortars  is  especially 
noticeable  in  the  leaner  mixtures. 

A  convenient  method  of  comparing  the  gradation  of  differ- 
ent sands  is  by  means  of  their  uniformity  coefficients.  To  de- 
termine this  coefficient,  a  sample  of  sand  is  screened  over  a  set 
of  screens  and  the  percentage  by  weight  of  the  sample  passing 
each  is  noted.     A  plot  is  then  made  of  this,  as  shown  in  Fig.  12. 


100 


75 


g  1 


25 


IP 


QUa 


yf 


100 


75 


50 


.025       .050      .075      .100       .125       .150       .175       .200      .225       .250 
Diameter  of  Particles  in  Inches 

Fig.  12.  —  Mechanical  Analysis  Curve  for  Blue  River  Sand. 

This  is  called  a  mechanical  analysis  curve.  It  is  made  by 
laying  off  to  scale  on  the  horizontal  lines  the  diameters  of  the 
meshes  of  the  sieves,  and  on  the  vertical  lines  the  percentages  of 
sand  passing  sieves  of  the  given  diameters  of  mesh.  A  smooth 
curve  is  then  drawn  through  the  points  found,  and  this  indi- 
cates approximately  the  percentage  of  particles  smaller  than 
any  given  size.  To  illustrate  the  plotting  of  the  curve,  the 
analysis  from  which  Fig.  12  was  plotted  is  shown  below: 


Sieve 

Diameter 

Per  cent  of 

No. 

of  mesh 

sample  passing 

V 

0.234 

100 

10 

0.081 

63 

16 

0.046 

42 

20 

0.036 

31 

30 

0.022 

10 

50 

0.013 

2.5 

100 

0.0067 

1.5 

38    CONCRETE  CONSTRUCTION  FOR   RURAL  COMMUNITIES 

After  the  curve  is  plotted,  the  diameter  of  the  mesh  through 
which  60  per  cent  of  the  sample  would  pass  and  the  diameter 
through  which  10  per  cent  would  pass  are  noted.  The  quotient 
of  the  former  divided  by  the  latter  is  known  as  the  uniformity 
coefficient.  This  term  may  therefore  be  defined  as  the  ratio  of 
the  diameter  of  the  particles  having  60  per  cent  smaller  than 
themselves  to  the  diameter  of  the  particles  having  10  per  cent 
smaller  than  themselves.  In  the  given  curve,  the  diameter  of 
mesh  through  which  60  per  cent  of  the  sample  would  pass  is  about 
0.075  inch,  while  that  through  which  10  per  cent  would  pass  is 
about  0.022  inch.  Hence  the  uniformity  coefficient  is  0.075  ■*■ 
0.022  =  3.4. 

A  high  value  of  the  uniformity  coefficient  indicates  a  good 
gradation  of  sizes  from  fine  to  coarse.  However,  there  may  be 
considerable  variation  in  the  coarseness  of  different  sands  hav- 
ing about  the  same  uniformity  coefficient,  and  hence  this 
quantity  alone  cannot  be  regarded  as  a  fair  index  of  the 
quality  of  the  sand.  A  high  uniformity  coefficient,  together 
with  a  high  value  for  the  size  of  particles  at  the  60  per  cent 
line,  indicates  a  well-graded  coarse  sand,  which  if  free  from 
injurious  impurities  is  well  suited  to  concrete  work. 

Impurities.  —  The  effect  of  impurities  in  the  sand  on  the 
strength  of  concrete  will  differ  greatly  with  the  nature  of  the 
impurities  and  the  richness  of  the  mixture.  It  seems  to  be 
well  established  that  a  considerable  amount  of  clay  or  other 
mineral  impurities,  in  some  cases  20  per  cent  or  more,  may  be 
present  in  lean  mortars,  such  as  1:3  or  1:4,  without  injury,  and 
that  a  small  amount  of  clay  may  even  increase  the  strength. 
On  the  other  hand,  in  richer  mixtures  such  as  1:1  or  1:2,  a 
much  smaller  percentage  of  clay  will  reduce  the  strength.  The 
explanation  seems  to  be  that  in  the  lean  mixtures  the  clay 
helps  to  fill  up  the  voids,  or  spaces  between  the  grains,  while  in 
the  richer  mixtures  there  is  more  than  enough  cement  paste  to 
fill  all  the  voids  in  the  sand. 

It  should  not  be  understood  from  this  that  it  is  desirable  to 
add  clay  to  any  sand,  but  only  that  sands  which  contain  a 
small  amount  of  clay  may  be  used  for  concrete,  if  clean  sand 
is  not  available.    A  clean  sand  should  always  be  chosen  in  pref- 


AGGREGATES  39 

erence  to  a  dirty  one,  and  sands  with  more  than  5  per  cent 
of  clay  should  not  be  used  without  first  being  tested. 

Dirty  sands  are  sometimes  washed  in  the  manner  described 
for  washing  gravel  on  page  43,  but  the  results  are  much  less 
satisfactory.  On  account  of  the  small  size  of  the  grains,  the 
particles  of  clay  and  silt  are  difficult  to  get  rid  of,  and  without 
special  washing  apparatus  it  is  not  usually  worth  while  to  try  it. 

Clay  which  is  present  in  the  form  of  balls  is  injurious,  as 
these  balls  have  about  the  same  effect  as  open  pockets  in  the 
concrete.  Most  of  them  can  be  removed  by  screening  the  sand 
through  a  J-inch  screen. 

Test  for  Impurities.  —  A  simple  test  to  determine  approxi- 
mately the  amount  of  clay  and  loam  in  the  sand  is  to  put  about 
four  inches  of  the  latter  into  a  quart  fruit  jar  and  then  to  fill 
the  jar  with  water  to  about  an  inch  from  the  top.  The  top 
should  then  be  fastened  on,  and  the  jar  shaken  vigorously  for 
several  minutes,  after  which  the  contents  should  be  allowed  to 
settle.  The  sand  will  settle  to  the  bottom,  with  the  clay  and 
loam  above  it.  The  dividing  line  can  be  seen  by  the  differ- 
ence in  color.  If  more  than  about  one-fourth  of  an  inch  of 
clay  and  loam  shows,  the  sand  should  not  be  used  without 
being  tested,  and,  if  possible,  a  cleaner  sand  should  be  obtained. 

A  more  accurate  test  can  be  made  by  washing  a  known 
weight  of  sand  through  several  waters  to  remove  all  the  silt, 
evaporating  the  water  and  weighing  the  residue. 

Organic  Impurities.  —  Vegetable,  or  organic,  impurities  in  the 
sand  are  extremely  injurious  even  in  very  small  quantities.  An 
amount  of  vegetable  loam  so  small  as  to  escape  notice,  except 
on  very  careful  examination,  may  render  sand  entirely  unfit  for 
concrete.  It  seems  to  form  a  coating  on  the  sand  grains  which 
prevents  the  adhesion  of  the  cement  and  retards  its  setting. 
Sand  suspected  of  containing  any  organic  matter  should  there- 
fore be  tested  before  being  used  in  work  of  any  importance. 

Those  sands  commonly  designated  as  "quicksands"  are  en- 
tirely unsuited  for  concrete  work,  as  they  are  very  fine  and  are 
often  coated  with  organic  matter. 

Strength  Test  for  Sand.  —  A  strength  test  of  mortar  made  with  a 
given  sand  is  the  final  test  as  to  whether  it  is  suitable  for  use  in 


40     CONCRETE  CONSTRUCTION  FOR   RURAL  COMMUNITIES 

concrete,  but  such  tests  can  be  made  only  in  laboratories  suitably 
equipped.  The  Joint  Committee  on  Concrete  from  the  national 
engineering  societies  recommends  the  following  requirements: 

"Fine  aggregates  should  be  of  such  quality  that  mortar  composed 
of  one  part  Portland  cement  and  three  parts  fine  aggregate  by  weight 
when  made  into  briquettes  will  show  a  tensile  strength  at  least  equal 
to  the  strength  of  1:3  mortar  of  the  same  consistency  made  with  the 
same  cement  and  standard  Ottawa  sand.1  If  the  aggregate  be  of  poorer 
quality  the  proportion  of  cement  should  be  increased  in  the  mortar  to 
secure  the  desired  strength. 

"If  the  strength  developed  by  the  aggregate  in  the  1:3  mortar  is 
less  than  70  per  cent  of  the  strength  of  the  Ottawa-sand  mortar,  the 
material  should  be  rejected.  To  avoid  the  removal  of  any  coating 
on  the  grains,  which  may  affect  the  strength,  bank  sand  should  not 
be  dried  before  being  made  into  mortar,  but  should  contain  natural 
moisture.  The  percentage  of  moisture  may  be  determined  upon  a 
separate  sample  for  correcting  weight.  From  10  to  40  per  cent  more 
water  may  be  required  in  mixing  bank  or  artificial  sands  than  for  stand- 
ard Ottawa  sand  to  produce  the  same  consistency." 

It  should  be  noted  that  this  is  given  as  the  minimum 
requirement  for  sand.  The  Ottawa  sand  is  adopted  as  a  stand- 
ard, not  because  it  is  an  unusually  good  sand,  but  because  it  is 
a  definite  sand,  uniform  in  its  properties,  and  is  available  any- 
where in  the  country.  It  is  used  only  as  a  sort  of  yardstick  by 
which  other  sands  are  measured.  Any  really  good  sand  will 
give  a  strength  at  least  equal  to  the  Ottawa  sand,  and  the  better 
sands  will  show  a  considerably  greater  strength. 

Stone  Screenings.  —  Stone  screenings  are  sometimes  used 
instead  of  sand  for  the  fine  aggregate  of  concrete.  A  screen 
of  about  J-inch  mesh  should  be  used.  The  stone  should  be 
reasonably  hard  and  the  screenings  should  be  free  from  im- 
purities and  an  excessive  amount  of  dust.  Screenings  from  any 
of  the  stones  suitable  for  the  coarse  aggregate  may  be  used. 
The  discussion  of  sand  will,  for  the  most  part,  apply  equally 
well  to  stone  screenings,  and  the  same  general  properties  are 
required;    namely,  cleanness  and  coarseness. 

Broken  Stone.  —  The  coarse  aggregate  used  in  concrete  work 
consists  usually  of  broken  stone,  gravel,  or  cinders.  Broken 
1  See  footnote  on  p.  16. 


AGGREGATES 


41 


stone  for  concrete  should  be  clean,  hard,  and  of  a  size  suited  to 
the  character  of  the  work  in  which  it  is  used.  Any  sound  stone, 
such  as  is  used  for  building  purposes,  may  be  used.  Trap, 
granite,  conglomerate,  hard  limestone,  and  hard  sandstone  are 
all  widely  used  and  are  well  suited  to  the  purpose.  Soft  lime- 
stones, soft  sandstones,  slate,  and  shale  should  not  be  used  if  it 
is  possible  to  obtain  hard  stone.  Broken  hard-burned  brick 
may  be  used,  but  softer  grades  of  brick  make  weak  concrete. 
A  stone  which  breaks  into  cubical  or  other  angular  shapes  is 
much  to  be  preferred  to  one  that  gives  thin,  flat  pieces. 

Size  of  Stone.  —  The  size  of  broken  stone  to  be  used  de- 
pends on  the  character  of  the  work.  For  thin  walls  or  for  rein- 
forced concrete,  in  which  there  is  an  intricate  network  of 
steel,  stone,  the  largest  particles  of  which  will  pass  through  a 
1-inch  or  smaller  ring,  should  be  used.  For  heavier  work  the 
size  may  be  increased  to  2  or  2 \  inches.  The  general  rule  may 
be  given  that  the  diameter  of  the  largest  pieces  of  stone  should 
not  be  greater  than  \  to  \  the  thickness  of  the  concrete.  As 
with  sand,  a  mixture  of  coarse 
and  fine  particles  will  give 
stronger  concrete  than  will  a 
stone  of  uniform  size. 

In  unimportant  work,  the 
stone  may  be  used  as  it  comes 
from  the  crusher  without 
screening,  if  the  amount  of 
fine  material  runs  fairly  uni- 
form and  is  not  excessive.  If 
this  is  done,  the  proportion 
of  sand  used  should  be  cut 
down  to  make  allowance  for 
the  fine  material  in  the  stone. 
It  will  often  pay,  if  a  dense    Fig.  13.  — Details  for  Construction  of  In- 

,.  .  clined  Screen  for  Stone  or  Gravel, 

waterproof     concrete    is    re- 
quired, or  if  the  fine  material  is  distributed  unevenly,  or  if  an  ex- 
cessive amount  of  clay  or  loam  is  present,  to  screen  the  stone  over 
a  i-inch  screen.    The  screenings  may  be  mixed  with,  and  used  as 
a  part  of,  the  sand,  if  they  are  clean.  -  Figure  13  gives  details  for 


42    CONCRETE  CONSTRUCTION  FOR  RURAL  COMMUNITIES 


constructing  a  suitable  screen.  If  large  quantities  of  material  are 
to  be  screened,  or  if  the  material  is  to  be  separated  into  several 
sizes,  it  will  be  desirable  to  have  a  revolving  screen,  such  as  is 
shown  in  Fig.  14. 


i 

1  '11 

Fig.  14.  —  Small  Portable  Revolving  Screen. 

Any  large  flat  pieces  of  stone  which  may  pass  through  the 
crusher  should  be  picked  out  by  hand. 

Gravel.  —  Gravel  can  frequently  be  obtained  more  cheaply  or 

conveniently  than  broken  stone.     If  it  is  clean  and  well  graded, 

it    will    make    as  good    concrete    as   the   stone.     Gravel,   as   it 

occurs  in  the  bank  or  the  creek  bed,  is  usually  mixed  with  a 

larger  or  smaller  amount  of  sand  than  is  suitable  for  concrete, 

and  the   different   sizes   are   usually   distributed   in   seams   and 

pockets.     It  should  therefore  be  screened  over  a  J-inch  screen,  the 

part  passing  through  being  used  as  sand.     If  larger  pebbles  are 

present  than  should    be  used,1  it  may  be    necessary  to  screen 

these  out  with  a  coarse  screen.    Thin,  flat  pieces  are  unsuitable 

and  should  be  thrown  out. 

1  See  the  discussion  of  the  size  of  particles  in  connection  with  the  subject 
of  broken  stone. 


AGGREGATES 


43 


Washing  of  Gravel.  —  Dirty  gravel  can  be  washed  by  spread- 
ing it  out  in  a  thin  layer  on  a  mixing  platform  which  has  one 
edge  elevated  six  inches  or  more  and  turning  a  hose  on  it,  or 
throwing  water  on  it  from  buckets.  The  remarks  made  about 
impurities  in  sand  apply  with  equal  force  to  impurities  in  gravel. 


Slope  5  Ft.  in  12  Ft 


1  In. Board; 


Trough  to  Kun  off  Dirty  Water 
to  be  Lined  with  Tar  Paper 


Fig.  15.  —  Trough  for  Washing  and  Screening  Sand  or  Gravel. 

Figure  15  shows  a  method  which  can  be  used  for  screening  and 
washing  dirty  gravel  or  sand.  For  sand,  a  screen  of  30  meshes 
to  the  inch  should  be  used  and  the  cleats  should  be  placed 
close  together  to  support  the  weight  of  the  sand.  For  gravel, 
use  a  J-inch  screen.  The  sand  or  gravel  is  thrown  on  the  upper 
end  of  the  screen  by  one  man,  while  another  throws  a  stream 
of  water  on  it  from  a  hose.  The  water  washes  the  sand  or  gravel 
down  the  screen,  while  the  dirty  water  drains  off  in  the  trough 
provided  for  this  purpose. 

Cinders.  —  Cinders  are  sometimes  used  for  concrete  where 
little  strength  is  required  and  where  the  concrete  acts  chiefly 
as  a  filler.  They  are  entirely  unsuitable  for  general  concrete 
work.  When  used,  they  should  be  free  from  dust,  ashes,  and 
particles  of  unburned  coal. 


PART  II 
PLAIN  CONCRETE 


CHAPTER  IV 
PROPORTIONS   AND    QUANTITIES    OF   MATERIALS 

The  Problem  in  Proportioning  Concrete.  —  Too  little  consid- 
eration is  generally  given  to  the  subject  of  proportioning,  es- 
pecially on  small  work.  A  little  cement,  sand,  and  stone,  or 
cement  and  sand  only,  are  often  mixed  together  without  any 
regard  for  the  proper  proportioning  or  accurate  measuring  of 
the  materials,  with  the  result  either  that  the  concrete  is  inade- 
quate for  its  purpose,  or  else  that  the  cost  for  materials,  espe- 
cially cement,  is  higher  than  is  necessary.  When  one  sees  the 
abuse  to  which  concrete  is  subjected  in  this  respect,  one  ceases 
to  wonder  that  it  is  not  always  satisfactory,  and,  instead,  is 
surprised  that  it  can  stand  up  as  well  as  it  does. 

The  proper  proportions  differ  with  the  materials  used,  and 
with  the  purpose  for  which  the  concrete  is  being  made.  The 
three  properties  which  are  most  often  required  are  (1)  strength, 
as  in  bridges  and  buildings;  (2)  resistance  to  abrasion  or  wear, 
as  in  concrete  sidewalks  and  roads;  and  (3)  impermeability,  or 
water-tightness,  as  in  water  tanks  and  silos.  The  problem  in 
proportioning  concrete  is  to  determine  the  amounts  of  cement, 
sand,  and  stone  which  must  be  used  in  order  to  obtain  these 
properties  at  the  least  expense  for  materials  and  labor.  Before 
one  can  understand  the  fundamental  principles  governing  the 
proportioning  of  materials,  one  must  consider  the  places  occupied 
by  the  different  materials  in  the  concrete. 

Voids.  —  The  voids,  or  open  spaces,  between  the  particles 
form  a  considerable  part  of  the  volume  of  a  material  such  as 
sand  or  broken  stone.  This  is  readily  understood  when  one 
realizes  that  a  large  amount  of  water  can  be  poured  into  a 
bucket  already  filled  with  these  materials.  In  broken  stone 
screened  to  a  uniform  size,  the  voids  will  be  about  50  per  cent 
of  the  total  volume,  and  in  screened  gravel  about  40  per  cent. 

47 


48     CONCRETE  CONSTRUCTION  FOR   RURAL   COMMUNITIES 


In  sand  the  voids  form  about  30  to  40  per  cent  of  the  volume. 
In  cement,  although  the  particles  are  very  fine  and  the  indi- 
vidual voids  are  small,  the  proportion  of  voids  to  the  whole 
volume  is  high,  being  about  50  per  cent  when  the  cement  is 
packed,  and  more  when  it  is  loose. 

When  these  materials  are  mixed  together  with  water  to  make 
concrete,  most  of  the  sand  goes  into  the  voids  of  the  stone,  and 
the  cement  paste  remaining  after  the  particles  of  sand  and  stone 
are  coated  goes  into  the  voids  of  the  sand.  The  result  is  that, 
while  the  volume  of  concrete  produced  is  not  much  greater  than 
the  volume  of  stone  used,  the  concrete  is  much  denser;  i.e., 
has  a  smaller  percentage  of  voids  than  any  of  the  separate 
materials  used.    This  is  illustrated  in  Fig.  16. 


I< 12  — 

Cement  Sand 

Fig.  16.  —  Diagram  Showing  Relation  of  Materials  to  Each  Other  in  Concrete. 


— Vt" *\ 

Concrete 


To  Find  the  Percentage   of  Voids  in  Coarse  Aggregate.  — 

The  voids  in  the  broken  stone  or  gravel  may  be  found  approxi- 
mately by  finding  how  much  water  can  be  poured  into  a  certain 
volume  of  the  material.  Fill  the  measure  level  full  with  the 
materials  to  be  tested,  weigh  it,  and  pour  in  water  until  the 
measure  will  hold  no  more.  Weigh  the  measure  again  and 
take  the  difference  in  the  two  weights.  This  is  the  weight  of 
water  required  to  fill  the  voids.  Now  empty  the  measure  and 
find  the  weight  of  water  required  to  fill  it.  Divide  the  weight  of 
water  required  to  fill  the  voids  by  this  weight,  and  the  quotient 
multiplied  by  100  will  be  the  percentage  of  voids  in  the  material 
tested.  The  result  is  likely  to  be  a  little  lower  than  the  true 
value,  on  account  of  air  trapped  between  the  particles,  but  the 
error  will  be  small.  More  nearly  accurate  results  may  be  ob- 
tained by  introducing  the  water  into  the  bottom  of  the  vessel 
through  a  pipe  than  by  pouring  it  in  at  the  top. 


PROPORTIONS  AND  QUANTITIES  OF  MATERIALS        49 

Problem: 

Weight  of  measure  filled  with  stone -  123  lb. 

Weight  of  measure  filled  with  stone  and  water =  155  lb. 

Weight  of  measure  filled  with  water =    84  lb. 

Weight  of  measure  empty =    13  lb. 

Find  percentage  of  voids  in  the  stone. 
Solution: 

155  -  123  =  32  lb.  =  Weight  of  water  required  to  fill  voids. 
84  -    13  =  71  lb.  -  Weight  of  water  required  to  fill  measure. 
\\  X  100  =  45         =  Percentage  of  voids  in  stone. 

To  Find  the  Percentage  of  Voids  in  Fine  Aggregate.  —  The 

foregoing  method  will  not  give  accurate  results  for  sand  because 
of  the  large  amount  of  air  entrapped.  Sometimes  a  somewhat 
similar  method  is  used,  the  sand  being  poured  into  the  water 
instead  of  water  into  the  sand.  A  better  and  simpler  method  is 
to  use  the  specific  gravity  of  the  sand. 

By  the  term  specific  gravity  is  meant  the  ratio  of  the  weight 
of  a  given  volume  of  any  material  to  the  weight  of  an  equal 
volume  of  water.  With  such  substances  as  sand,  consisting  of 
a  large  number  of  individual  particles,  the  volume  meant  in  this 
definition  is  the  actual  volume  occupied  by  the  particles  them- 
selves, not  including  the  spaces  between  the  particles.  The  spe- 
cific gravity  of  all  sands  used  for  concrete  work  is  practically 
constant  and  is  2.65.  This  being  known,  it  becomes  a  simple 
matter  to  find  the  percentage  of  voids  in  a  given  sand.  All  we 
need  to  do  is  to  find  the  weight  of  a  given  volume  of  the  sand 
and  compare  this  with  the  weight  of  the  same  volume  of  solid 
material. 

To  use  this  method,  we  may  fill  any  convenient  measure 
with  sand  in  the  condition  in  which  we  wish  to  find  the  per- 
centage of  voids.  This  sand  is  then  dried  at  a  temperature  of 
not  less  than  212°  F.  and  is  weighed.  The  weight  of  water  re- 
quired to  fill  the  vessel  (with  no  sand  in  it)  is  now  found.  The 
latter,  multiplied  by  2.65,  gives  the  weight  of  an  equal  volume 
of  solid  sand  (no  voids).  '  Subtract  from  this  the  actual  weight 
of  sand  found,  divide  the  result  by  the  weight  of  the  solid 
sand,  and  multiply  the  quotient  by  100.  The  result  is  the 
percentage  of  air  and  moisture  voids  in  the  original  sample  of 
sand.     If   the   percentage   of  air  voids   alone   is   required,    the 


50     CONCRETE  CONSTRUCTION  FOR   RURAL  COMMUNITIES 

percentage  of  the  volume  occupied   by  the  moisture  may  be 
found  and  subtracted  from  the  total  voids. 

Problem:  —  The  sand  required  to  fill  a  given  vessel  weighs  122  oz.,  and  the 
weight  after  drying  is  117  oz.,  while  the  weight  of  water  required  to  fill  the 
same  vessel  is  67  oz.     Find  the  percentage  of  voids. 

Solution.  —  The  weight  of  the  vessel  full  of  solid  sand  (no  voids)  is 

67  x  2.65  -  177.6  oz. 

Hence,  the  difference  between  weight  of  vessel  full  of  solid  sand  and  weight 
of  given  sand  required  to  fill  the  vessel  is 

177.6  -  117  -  60.6  oz. 

fin  fi 
and  there  is  — — '—  x  100  =  34.1  per  cent 

177.6 

of  air  and  water  voids  in  the  sample  of  sand.    The  weight  of  water  in  the 
sand  was 

122  -  117  =  5  oz. 

hence  there  is  —  X  100  =  7.5  per  cent 

of  water  voids,  and  34.1  -  7.5  =  26.6  per  cent 

of  air  voids  in  the  sand. 

Proportions  for  Maximum  Density.  —  Since  most  of  the  sand 
particles  go  into  the  voids  of  the  broken  stone,  it  is  easily  seen 
that  a  mixture  of  sand  and  stone  in  the  proper  proportions  will 
contain  a  smaller  percentage  of  voids,  and  hence  will  have  a 
greater  density  than  either  of  the  materials  separately.  The 
greatest  density  will  be  obtained  when  the  least  amount  of  sand 
is  used  which  will  fill  all  the  spaces  between  the  pieces  of  stone 
in  the  mixture.  It  must  be  remembered,  however,  that  some 
of  the  sand  particles  will  get  between  the  pieces  of  stone,  hold- 
ing them  apart  slightly  and  thus  increasing  the  percentage  of  stone 
voids  over  what  it  would  be  with  no  sand  present.  Hence  the 
percentage  of  sand  used  must  be  a  little  greater  than  the  per- 
centage of  voids  in  the  stone.  Likewise,  the  densest  concrete 
will  be  produced  by  using  an  amount  of  mortar  just  sufficient 
to  fill  the  spaces  between  the  stones  6r  a  little  more  than  the 
percentage  of  voids  in  the  broken  stone.  The  usual  allowance 
of  excess  is  about  10  per  cent  by  volume. 

Fundamental  Laws  of  Proportioning.  —  Experiments  have 
shown  that 


PROPORTIONS  AND  QUANTITIES  OF  MATERIALS        51 

(1)  With  the  same  percentage  of  cement,  the  concrete  will  be 
strongest  and  most  impermeable  when  the  fine  and  coarse 
aggregates  are  so  proportioned  to  each  other  as  to  give  the 
greatest  density; 

(2)  With  the  same  aggregates  in  the  same  proportions  to 
each  other,  that  concrete  will  be  strongest  and  most  imper- 
meable which  contains  the  greatest  percentage  of  cement. 

It  is  generally  true  that  the  concrete  which  is  strongest  and 
most  impermeable  will  also  best  resist  wear  or  abrasion,  so  that 
the  above  laws  are  fundamental  for  all  classes  of  concrete  work. 

It  follows  from  these  laws  that  we  should  use  only  enough 
sand  to  fill  all  the  voids  in  the  broken  stone  with  mortar  (mak- 
ing due  allowance  for  the  unavoidable  separation  of  the  particles 
of  stone  by  the  sand  grains  which  get  between  them)  and  that 
we  should  use  enough  cement  with  this  mixture  of  sand  and 
stone  to  give  the  strength,  impermeability,  or  resistance  to 
abrasion  that  we  desire.  If  the  amount  of  sand  used  is  either 
more  or  less  than  enough  to  fill  the  stone  voids  with  mortar,  the 
concrete  will  be  less  dense,  and  more  cement  will  be  required  to 
give  the  same  strength.  This  is  one  important  reason  why  it  is 
desirable  to  screen  the  fine  material  out  of  broken  stone  and 
gravel,  and  then  remix  the  materials  in  the  proper  proportion 
of  fine  to  coarse,  or  at  least  to  reduce  the  amount  of  sand  to 
allow  for  the  fine  material  which  may  be  present  in  the  stone 
or  gravel.  The  amount  of  cement  which  can  be  saved  by 
screening  the  materials  will  usually  more  than  pay  for  the 
labor  of  screening,  unless  the  materials  run  very  uniform. 

Arbitrarily  Specified  Proportions.  —  As  the  percentage  of  voids 
in  different  lots  of  screened  broken  stone  or  gravel  will  not 
differ  very  greatly,  it  is  customary  in  much  construction  work 
to  specify  definite  ratios  of  the  materials,  without  special  refer- 
ence to  the  particular  materials  to  be  used  in  any  given  instance. 
If  the  materials  are  of  good  quality  and  of  average  character, 
this  works  out  very  well,  and  it  is  doubtful  if  the  method  can 
be  improved  upon  for  small  work. 

The  voids  in  broken  stone  with  the  dust  screened  out  aver- 
age about  45  per  cent.  In  gravel  screened  over  a  J-inch 
screen  the  voids  run  about  40  per  cent.    With  allowance  for  the 


52     CONCRETE  CONSTRUCTION  FOR   RURAL   COMMUNITIES 

separation  of  the  coarse  aggregate  by  the  particles  of  sand,  for 
the  lack  of  perfect  uniformity  in  the  distribution  of  the  coarse 
and  the  fine  particles,  and,  on  the  other  hand,  with  considera- 
tion of  the  fact  that  the  cement  and  water  will  slightly  increase 
the  volume  of  the  mortar  over  that  of  the  sand,  it  will  be 
approximately  correct  to  use  an  amount  of  sand  equal  to  one- 
half  the  volume  of  the  stone  or  gravel.  This  ratio  will  be  nearly 
correct,  irrespective  of  the  amount  of  cement  used,  except  in 
rich  mixtures.  For  ratios  of  cement  to  sand  greater  than  1:2, 
the  ratio  of  sand  to  stone  should  be  somewhat  decreased. 

If  the  stone  or  gravel  has  not  been  well  screened,  the  amount 
of  sand  may  be  decreased  a  little  from  the  ratio  given  above. 
If,  when  the  concrete  is  well  spaded  or  tamped,  more  mortar 
flushes  to  the  top  than  is  sufficient  to  cover  all  the  stones, 
then  less  sand  may  be  used;  but,  if  it  is  difficult  to  get  any 
mortar  to  flush  to  the  top,  a  little  more  sand  should  be  used. 

The  following  proportions  are  much  used  in  concrete  work. 

A  rich  mixture  of  1  part  of  cement,  1|  parts  of  sand,  and  3 
parts  of  broken  stone  or  gravel,  usually  known  as  a  1:1$:  3 
mix,  is  used  for  columns  of  reinforced  concrete  buildings,  for 
thin  walls  that  must  be  water-tight,  and  wherever  a  very  strong, 
dense  concrete  is  required.  The  mixture  is  richer  than  is  re- 
quired for  most  work  and  is  to  be  considered  exceptional.  In 
most  cases  the  concrete  would  be  improved  by  reducing  the 
amount  of  sand  to  1: 1J:3  or  1:1:3. 

A  good  mixture  of  1  part  of  cement,  2  parts  of  fine  aggregate, 
and  4  parts  of  coarse  aggregate,  known  as  a  1:2:4  mix,  is 
used  for  reinforced  concrete  work  of  all  kinds,  for  water  tanks, 
for  thin  walls,  and  for  any  purpose  where  a  good  concrete  is 
required,  with  considerable  strength  and  impermeability.  This 
and  the  following  mixture  are  the  ones  best  adapted  for  gen- 
eral concrete  work. 

A  medium  mixture  of  1  part  of  cement,  2J  parts  of  sand,  and  5 
parts  of  broken  stone  is  used  for  plain  concrete  l  work  of  all  kinds, 
for  foundations,  for  walls,  for  floors,  and  for  all  other  purposes  for 
which  a  good  concrete  is  required,  but  not  so  much  strength  nor 
impermeability  is  necessary  as  to  require  a  1:2:4  mix. 
1  Concrete  not  reinforced  by  steel  rods. 


PROPORTIONS  AND  QUANTITIES  OF  MATERIALS         53 

A  lean  mixture  of  1  part  of  cement,  3  parts  of  fine  aggregate, 
and  6  parts  of  coarse  aggregate,  known  as  a  1:3:6  mix,  is  used 
in  heavy  masses  where  the  loads  are  wholly  compressive  and 
of  moderate  intensity,  and  where  the  principal  requirements  are 
weight  and  stability  combined  with  a  moderate  degree  of 
strength,  as  in  heavy  walls,  foundations,  and  bridge  piers. 

Still  leaner  mixtures  are  sometimes  used,  but  they  are  not 
recommended  for  general  use. 

Other  Methods  of  Proportioning. — Of  the  more  scientific 
methods  of  proportioning,  but  two  will  be  mentioned: 

(1)  Use  of  trial  mixtures  to  determine  proportions  for  maximum 
density. 

(2)  Use  of  a  mechanical  analysis  curve. 

On  large  jobs  the  saving  of  cement  will  more  than  repay  the 
trouble  and  expense  of  determining  the  best  proportions  by  one 
of  these  methods. 

Trial  Mixtures.  —  A  strong,  rigid  vessel,  such  as  a  piece  of 
steel  pipe,  8  or  10  inches  in  diameter  and  2  feet  high,  is  re- 
quired for  the  application  of  this  method.  Weigh  out  the  ma- 
terials in  the  proportions  thought  desirable,  using  such  amounts 
as  will  make  enough  concrete  to  fill  the  cylinder  nearly  to  the 
top.  Mix  the  materials  on  a  non-absorbent  surface,  such  as  a 
piece  of  sheet  iron,  and  tamp  the  mixture  into  the  cylinder, 
noting  carefully  the  height  to  which  it  is  filled.  It  is  well  to 
weigh  the  cylinder  before  and  after  filling  it,  as  a  check  on  the 
amount  of  materials  used  and  the  amount  lost  in  handling. 

Now  throw  out  this  batch,  clean  the  cylinder,  and  mix  up 
another  batch,  using  the  same  weights  of  cement  and  water 
and  the  same  total  weight  of  sand  and  stone  as  before,  but 
making  the  ratio  of  sand  to  stone  slightly  different.  Note 
whether  this  batch  fills  the  cylinder  to  a  higher  or  a  lower 
point  than  the  first  batch.  If  it  fills  it  higher,  it  is  a  poorer 
mixture,  and  if  lower,  it  is  a  better  mixture  than  the  first.  Con- 
tinue making  trial  batches  until  the  proportion  is  found  which 
gives  the  least  height  in  the  cylinder.  This  will  be  the  best 
which  can  be  obtained  with  the  given  materials,  using  the 
given  amount  of  cement,  since  it  will  make  the  densest  concrete. 

Mechanical  Analysis. — This  method  of  proportioning  is  recom- 


54     CONCRETE  CONSTRUCTION  FOR  RURAL   COMMUNITIES 

mended  and  used  by  some  of  the  best  concrete  engineers  in 
the  country.  It  requires  special  apparatus  and  is  not  generally 
applicable  for  small  work,  but  is  mentioned  here  for  the  light 
it  throws  on  the  proper  grading  of  the  size  of  particles  for  the 
best  concrete.1  Extensive  tests  have  shown  that  the  best  prac- 
tical mixture  is  one  in  which  the  particles  are  so  graded  from 
fine  to  coarse  that  a  mechanical  analysis  curve,  similar  to  the 
one  shown  in  Fig.   12,  but   made    to    include  the  cement  and 


1UU 

90 

^ — " 

9  80 

$  70 

1  60 

&* 

|  30 

y 

' 

/ 

o 

/ 

g  20 

/ 

cu 

10 

0      .1       .2       .3       .4      .5      .6       .7       .8      .9      1.0     1.1     1.2     1.3     1.4    1.5 
Diameter  of  Particles  in  Inches 

Fig.  17.  —  Maximum  Density  Curve  with  Stone  Screened  Through 
H-Inch  Screen. 

broken  stone  as  well  as  the  sand,  follows  most  closely  a  certain 
curve  known  as  a  maximum  density  curve.  This  maximum 
density  curve  is  a  combination  of  an  ellipse  and  a  straight  line 
and  can  easily  be  plotted  for  any  given  materials.  Figure  17 
shows  such  a  curve. 

As  proof  of  the  efficiency  of  this  method,  it  is  stated  that 
water-tight  concrete  has  repeatedly  been  secured  with  as  lean 
a  mixture  as  1:3:7,  whereas  the  ordinary  mixture  for  water- 
tight concrete  is  about  1:  2:  4.    The  saving  in  cement  is  evident. 

Omission  of  Coarse  Aggregate.  —  It  is  often  convenient,  and 
sometimes  desirable,  to  omit  the  coarse  aggregate  from  con- 
crete. Sidewalks  and  floors  are  often  made  in  this  way,  as  are 
also  thin  walls,  building  blocks,  brick,  tile,  and  other  objects. 

1  For  a  full  discussion  of  the  method,  the  reader  is  referred  to  Taylor  and 
Thompson's  "Concrete,  Plain  and  Reinforced,"  John  Wiley  and  Sons,  New 
York. 


PROPORTIONS  AND   QUANTITIES  OF  MATERIALS        55 

When  this  is  done,  a  little  larger  proportion  of  sand  may  be 
used  than  in  the  usual  cement,  sand,  and  stone  concrete,  but 
the  number  of  parts  of  sand  should  be  made  less  than  the 
number  of  parts  of  stone  commonly  used  in  such  a  mixture. 
For  example,  a  mixture  of  1 : 4  might  be  used  in  a  place  where 
concrete  of  about  1 :  2 \ :  5  quality  is  required.  This  concrete 
will  usually  be  a  little  weaker  and  less  dense  than  stone  con- 
crete, and  a  mixture  of  1:3^  would  be  better. 

It  will  usually  be  cheaper  to  make  concrete  of  a  given  strength 
with  stone  or  gravel  than  with  sand  alone.  Tests  made  in  the 
laboratories  of  the  Kansas  State  Agricultural  College  indicate 
that  it  may  be  economical  to  pay  as  much  as  $2.50  per  yard 
for  stone,  rather  than  to  use  enough  extra  cement  to  get  the 
same  strength  with  cement  and  sand  only. 

Use  of  Bank-run  Gravel.  —  Sand  and  gravel  are  sometimes 
used  mixed  as  they  come  from  the  bank.  This  is  seldom  eco- 
nomical, as  there  will  almost  always  be  an  excess  of  sand.  Even 
if  these  materials  are  mixed  in  just  the  right  proportions,  it 
should  be  remembered  that  the  sand  has  not  increased  the 
volume  of  the  gravel  to  any  great  extent.  Hence  the  volume 
of  mixed  sand  and  gravel  used  with  a  given  amount  of  cement 
should  be  no  greater  than  would  be  required  of  the  gravel  only, 
if  the  sand  were  to  be  added  separately.  Thus,  a  mixture  of  1 
part  of  cement  to  5  parts  of  a  well-graded  bank-run  gravel  is 
in  reality  approximately  equivalent  to  1  part  of  cement,  2\ 
parts  of  sand,  and  5  parts  of  gravel,  the  sand  grains  being  pres- 
ent in  the  voids  of  the  gravel.  With  the  excess  of  sand  usually 
present,  not  more  than  4  or  4|  parts  of  the  gravel  can  be  used 
with  1  part  of  cement  to  get  as  good  concrete  as  a  1:2^:5. 

If  the  sand  and  gravel  run  very  uniform  and  are  not  in  the 
desired  proportions,  a  small  amount  may  be  screened  with  a 
J-inch  screen  and  the  proportions  of  sand  and  gravel  determined. 
An  additional  amount  of  screened  gravel  may  then  be  added 
to  the  bank-run  gravel  to  give  the  desired  proportions. 

Suppose,  for  example,  that  when  3  cubic  feet  of  a  bank-run 
gravel  is  screened,  it  is  found  to  contain  2  cubic  feet  of  gravel  and 
1.8  cubic  feet  of  sand,  and  it  is  desired  that  the  concrete  shall  con- 
tain twice  as  much  gravel  as  sand.     Then  to  each  3  cubic  feet 


56      CONCRETE  CONSTRUCTION  FOR  RURAL  COMMUNITIES 

of  bank-run  gravel  it  will  be  necessary  to  add  1.6  cubic  feet  of 
screened  gravel,  making  a  total  of  3.6  cubic  feet  of  gravel  to  cor- 
respond with  the  1.8  cubic  feet  of  sand.  It  will  be  necessary  to 
use  0.9  cubic  feet  of  cement  with  this  for  a  1:2:4  mix,  since  half 
as  much  cement  as  sand  is  required.  As  one  bag  of  cement  may 
be  considered  equal  to  a  cubic  foot,  we  must  use  for  each  bag 
of  cement  3  ■*■  0.9  =  3.3  cubic  feet  of  the  bank-run  gravel,  and 
i  ft 
^r  =  1.8  cubic  feet  of  screened  gravel. 

Quantities  of  Materials  Required.  —  In  estimating  the  quan- 
tities of  materials  required  for  a  job,  one  must  not  make  the 
mistake  of  assuming  that  one  cubic  yard  of  cement,  sand,  and 
stone  will  make  a  cubic  yard  of  concrete.  As  has  been  pre- 
viously pointed  out,  most  of  the  cement  and  sand  go  to  fill  up 
the  spaces  between  the  pieces  of  stone,  so  that  with  ordinary 
mixtures  the  volume  of  the  concrete  formed  is  not  much  greater 
than  the  volume  of  the  stone  used.  Neglecting  to  consider 
this  point  will  cause  a  very  serious  error  in  the  estimate  of  the 
quantities  of  materials  required,  and  will  result  in  a  considerable 
increase  in  the  cost  of  the  structure  over  the  estimated  amount. 

A  simple  method  of  estimating  the  quantities  of  materials 
required,  sufficiently  accurate  for  most  small  jobs  where  about 
half  as  much  sand  as  stone  is  used,  is  to  figure  on  a  volume  of 
stone  equal  to  the  volume  of  concrete  required,  and  then  de- 
termine the  quantities  of  sand  and  cement  from  the  relative 
parts  used  in  the  mixture.     Thus  for  8  cubic  yards  of  1 :  2§ : 5 

2\ 
concrete,  8  cubic  yards  of  stone  would  be  allowed,  —  or  \  as 

o 

much  sand  as  stone,  or  4  cubic  yards,  and  \  as  much  cement  as 

stone,  which  is  -§■  cubic  yards,  or  43^  cubic  feet.     As  one  bag  of 

cement  is  taken  as  1  cubic  foot,  this  is  equivalent  to  about  43 

bags  of  cement. 

When  sand  or  bank-run  gravel  alone  is  used,  it  is  obvious  that 

by  the  foregoing  method  we  should  allow  as  many  cubic  yards  of 

sand,  or  of  gravel,  as  there  are  cubic  yards  of  concrete  required, 

and  a  proportionate  amount  of  cement.     Thus  for  8  cubic  yards  of 

1:4  concrete  there  would  be  allowed  8  cubic  yards  of  bank- run 

gravel  and  2  cubic  yards,  or  54  cubic  feet,  or  bags,  of  cement. 


PROPORTIONS  AND  QUANTITIES  OF  MATERIALS         57 

This  method  will  usually  give  a  small  excess  amount  of  ma- 
terials because  the  volume  of  concrete  is  a  little  greater  than  that 
of  the  coarse  aggregate.  In  rich  mixtures  the  inaccuracy  will  be 
greater  than  in  ordinary  mixtures,  on  account  of  the  larger  amount 
of  cement  used  over  that  required  to  fill  the  voids.  When  there 
is  a  considerable  excess  of  sand,  as  in  a  1 : 3 : 4  mix,  this  method 
will  give  large  errors  and  should  not  be  employed. 

Fuller's  Rule.  —  A  more  nearly  accurate  method  of  estimat- 
ing quantities,  which  takes  into  account  the  variation  in  the 
richness  of  the  mixture,  and  the  increase  in  the  volume  of  the 
concrete  over  that  of  the  coarse  aggregates,  is  the  following, 
adapted  from  Fuller's  Rule:1 

To  find  the  number  of  bags  of  cement  required  far  each  cubic 
yard  of  concrete,  divide  42  by  the  sum  of  the  parts  of  the  materials, 
the  number  of  parts  of  cement  being  expressed  as  1.  The  amounts 
of  the  other  materials  can  be  then  found  from  th  eratio  of  their 
volume  to  that  of  the  cement. 

The  rule  can  be  expressed  algebraically  as  follows: 

Let  c    =  number  of  parts  of  cement  (  =1), 
s    =  number  of  parts  of  sand, 
g    =  number  of  parts  of  gravel, 
C  =  number  of   bags   of  cement   required   for 

each  cubic  yard  of  concrete, 
S  =  number  of  cubic  yards  of  sand  required 

for  each  cubic  yard  of  concrete, 
G  =  number  of  cubic  yards  of  gravel  required 

for  each  cubic  yard  of  concrete. 

42 
Then      C  = 


S  = 
G  = 


c  +  s  +  g 

CXs 

27 

CXg 

27 


1  For  the  original  form  of  this  rule,  see  Taylor  and  Thompson's  "Con- 
crete, Plain  and  Reinforced,"  2d  edition,  p.  16. 


58       CONCRETE  CONSTRUCTION  FOR  RURAL  COMMUNITIES 

By  this  rule,  the  quantities  of  materials  required  for  8  cubic 
yards  of  1 :  2 J :  5  concrete  are  found  as  follows.  For  each  yard 
of  concrete  there  will  be  required 

42 

=  4.94  bags  of  cement, 


l+2|  +  5 
4.94  X  2| 

27 

4.94  X  5 

27 


=  0.46  cubic  yards  of  sand,  and 
=  0.92  cubic  yards  of  stone. 


For  8  cubic  yards  of  concrete  the  amount  of  each  material 
will  be  8  times  as  great,  or  39.5  bags  of  cement,  3.68  cubic 
yards  of  sand  and  7.36  cubic  yards  of  stone.  These  values  are 
slightly  less  than  those  found  by  the  more  roughly  approximate 
method. 

In  applying  any  rule,  one  should  bear  in  mind  that  the 
volume  of  materials  required  will  vary  somewhat  with  the 
amount  of  fine  material  in  the  stone  or  other  coarse  aggregate. 
If  the  stone  is  screened  to  a  uniform  size,  about  5  per  cent 
more  of  all  materials  will  be  required  than  is  given  by  Fuller's 
Rule,  while  if  the  stone  contains  a  considerable  amount  of  small 
particles,  the  quantities  given  by  the  rule  may  be  decreased  by 
about  5  per  cent. 

Tables  of  Quantities.  —  A  still  more  exact  method  of  deter- 
mining the  quantities  of  materials  required  is  by  means  of 
tables  which  have  been  carefully  worked  out  and  checked  by 
actual  experience.  This  method  has  the  additional  value  of 
requiring  less  computation. 

The  table  on  the  opposite  page  applies  to  ordinary  materials 
and  covers  the  usual  range  of  proportions.  To  determine  by 
this  table  the  quantities  of  materials  for  8  cubic  yards  of  1 :  2 \ :  5 
concrete,  with  screened  stone,  we  find  that  for  each  cubic  yard 
of  concrete  there  will  be  required  5  bags  of  cement,  0.46  cubic 
yards  of  sand,  and  0.92  cubic  yards  of  stone;  then  for  8  cubic 
yards  of  concrete  the  quantities  required  will  be  8  X  5  =  40  bags 
of  cement,  8  X  0.46  =  3.68  cubic  yards  of  sand,  and  8  X  0.92  = 
7.36  cubic  yards  of  stone.  These  values  are  in  close  agreement 
with  those  value  found  by  Fuller's  Rule. 


PROPORTIONS  AND  QUANTITIES  OF  MATERIALS        59 


Table  IV 
Quantities  of  Materials  for  One  Cubic  Yard  of  Rammed  Concrete 


Quantities  required 

Quantities  required  for 

Proportions  by 

volume 

for  broken  stone  with 

crusher-run  stone  or 

dust  screened  out 

screened  gravel 

Packed 

Loose 

Loose 

Cement 

Sand 

Stone 

Cement 

Sand 

Stone 

cement 

sand 

st  one 

bags 

cu.  yd. 

cu.  yd. 

bags 

cu.  yd. 

cu.  yd. 

0 

0 

33.24 

33.24 

0    , 

19.5 

0.72 

19.5 

0.72 

H 

11.9 

0.44 

0.66 

11.5 

0.42 

0.64 

2 

10.5 

0.39 

0.78 

10.1 

0.37 

0.75 

2§ 

9.4 

0.35 

0.87 

9.0 

0.33 

0.83 

1 1 

x2 

0 

15.5 

0.86 

15.5 

0.86 

1  1 

A2 

2 

9.2 

0.51 

0.68 

8.9 

0.49 

0.66 

1  1 
A2 

3 

7.6 

0.42 

0.85 

7.3 

0.41 

0.81 

11 
*3 

4 

6.6 

0.36 

0.97 

6.2 

0.35 

0.92 

2 

0 

12.8 

0.95 

12.8 

0.95 

2 

3 

7.0 

0.52 

0.77 

6.7 

0.50 

6.74 

2 

4 

6.0 

0.45 

0.89 

5.8 

0.43 

0.85 

2 

5 

5.3 

0.39 

0.98 

5.0 

0.37 

0.93 

2h 

0 

11.0 

1.01 

11.0 

1.01 

2h 

4 

5.6 

0.51 

0.82 

5.3 

0.49 

0.79 

n 

5 

5.0 

0.46 

0.92 

4.7 

0.44 

0.87 

21 

6 

4.5 

0.41 

1.00 

4.2 

0.39 

0.94 

3 

0 

9.6 

1.06 

9.6 

1.06 

3 

4 

5.2 

0.58 

0.77 

5.0 

0.56 

074 

3 

5 

4.6 

0.52 

0.86 

4.4 

0.49 

0.82 

3 

6 

4.2 

0.47 

0.94 

4.0 

0.45 

0.90 

3 

7 

3.9 

0.43 

1.00 

3.7 

0.41 

0.95 

4 

0 

7.6 

1.13 

7.6 

1.13 

4 

7 

3.5 

0.52 

0.91 

3.3 

0.49 

0.86 

4 

8 

3.2 

0.48 

0.96 

3.1 

0.46 

0.91 

4 

9 

3.0 

0.45 

1.01 

2.9 

0.43 

0.96 

5 

0 

6.3 

1.17 

6.3 

1.17 

5 

10 

2.6 

0.49 

0.98 

2.5 

0.47 

0.93 

6 

0 

5.4 

1.20 

5.4 

1.20 

7 

0 

4.7 

1.22 

4.7 

1.22 

8 

0 

4.2 

1.24 

4.2 

1.24 

Note.  —  Variations  in  the  fineness  of  the  sand  and  the  compacting  of  the 
concrete  may  affect  the  quantities  by  10  per  cent  in  either  direction. 

1  Adapted   from  Taylor   and   Thompson's   "Concrete,   Plain  and  Rein- 
forced," 2d  edition  p.  232. 


60    CONCRETE  CONSTRUCTION   FOR  RURAL   COMMUNITIES 

PROBLEMS 

1.  A  certain  measure  weighs  8  pounds  when  empty.  When  filled  level 
full  of  water,  it  weighs  56  pounds;  when  filled  with  gravel,  83  pounds.  When 
water  is  poured  into  the  gravel  to  fill  the  voids,  the  total  weight  is  102  pounds. 
Find  the  percentage  of  voids  in  the  gravel. 

Ans.  39.6%. 

2.  Find  the  percentage  of  air  and  water  voids  in  a  sand,  if  a  certain  vessel 
holds  38  pounds  of  the  sand,  and  the  same  vessel  holds  20.9  pounds  of  water. 
The  sand  in  the  vessel  loses  1.2  pounds  in  weight  when  dried  at  212°  F. 

Ans.  33.6  %   total  voids. 

5.7  %  water  voids. 

27.9  %       air  voids. 

3.  In  a  test  of  certain  materials  by  the  method  of  trial  mixtures,  different 
batches  filled  the  cylinder  to  within  distances  from  the  top  as  given  in  the 
table.  What  are  the  best  proportions  for  the  materials  when  one  is  using  one 
part  cement  to  a  total  of  seven  parts  of  sand  and  gravel? 


Mix 

Distance  from  top  of 

cylinder  to  concrete 

1:21:41 

1" 

1:2}:  4| 

1|" 

l:2f:4f 

2" 

l:2J:4f 

2§" 

1:2*: 4f 

2" 

1:2:5 

U" 

Ans.  l:2£:4f 

4.  If  a  certain  bank-run  gravel  is  screened  over  a  j-inch  screen,  and  each 
cubic  foot  is  found  to  contain  0.7  cu.  ft.  of  gravel  and  0.6  cu.  ft.  of  sand,  how 
much  of  this  gravel  and  how  much  screened  gravel  must  be  used  with  each 
sack  of  cement  to  make  a  1 :  2 :  4  concrete? 

Ans.  Use  0.5  cu.  ft.  of  screened  gravel  for  each  cubic  foot  of  bank-run 
gravel.  For  each  sack  of  cement  use  3£  cu.  ft.  of  bank-run  gravel  and  If  cu. 
ft.  of  screened  gravel. 

5.  If  a  stone  from  a  crusher  is  found  to  contain  one  cubic  foot  of  screened 
stone  and  0.12  cu.  ft.  of  screenings  for  each  cubic  foot,  how  much  sand  and 
how  much  crusher-run  stone  must  be  used  with  each  bag  of  cement  to  make 
al:2|:5  concrete? 

Ans.   1.9  cu.  ft.  of  sand  and  5  cu.  ft.  of  crusher-run  stone. 

6.  Find  the  quantities  of  materials  required  for  12  cu.  yd.  of  1  :  2  :  5  con- 
crete by  each  of  the  three  methods  given  in  this  chapter. 

Ans.  By  approximate  method,  65  bags  cement,  4.8  cu.  yd.  sand,  and  12 
cu.  yd.  stone.    By  Fuller's  Rule,  63  bags  cement,  4.7  cu.  yd.  sand,  and  11.7 


PROPORTIONS  AND  QUANTITIES  OF  MATERIALS        61 

cu.  yd.  stone.    By  table,  63.6  bags  cement,  4.7  cu.  yd.  sand,  and  11.8  cu.  yd. 
stone. 

In  the  following  problems,  use  the  table  of  quantities. 

7.  Find  the  quantities  of  materials  required  for  a  sidewalk  160  feet  long  and 
5  feet  wide,  if  the  base  is  made  4  inches  thick  of  1:3:4  gravel  concrete,  and 
the  top  is  |  inch  thick  of  1 :  2  mortar. 

Ans.  For  top  coat,  23.7  bags  cement,  1.8  cu.  yd.  sand.  For  base,  49.5 
bags  cement,  5.5  "cu.  yd.  sand,  and  7.3  cu.  yd.  gravel. 

Total,  73.2  bags  cement,  7.3  cu.  yd.  sand,  and  7.3  cu.  yd.  gravel. 

8.  Find  the  cost  of  materials  for  the  walk  of  Problem  7  if  cement  costs 
45  cents  per  bag;  sand,  $0.75  per  load  of  one  and  one-half  yards;  and  screened 
gravel,  $1  per  yard,  all  delivered  on  the  site  of  the  work. 

Ans.  $43.89  or,  allowing  for  uncertainty  as  to  exact  amounts  of  materials, 
say  about  $45. 

9.  Find  the  cost  of  the  concrete  materials  for  a  circular  water  tank  8  feet 
inside  diameter  by  2\  feet  high,  with  sides  5  inches  thick  and  floor  6  inches 
thick.  All  concrete  is  mixed  1:2:4,  using  screened  gravel.  Use  the  same 
prices  as  in  Problem  8. 

10.  Find  the  cost  of  the  materials  for  a  concrete  wall  4  feet  high,  20  feet 
long,  and  12  inches  thick,  using  the  materials  of  Problem  4,  so  proportioned  as 
to  get  an  actual  1:3:6  concrete.    Use  the  prices  of  Problem  8. 

11.  How  much  could  be  saved  in  the  cost  of  a  cubic  yard  of  gravel  concrete 
by  using  a  1:2:5  mixture  instead  of  a  1:2:4  mixture?  Would  the  cost  be 
more  or  less  than  for  a  1:2^:5  mixture,  and  how  much?  Use  the  prices  of 
Problem  8. 


CHAPTER  V 
CONSTRUCTION   OF  FORMS 

Concrete  is  a  plastic  material  and  will  take  the  shape  of  any 
mold  or  container  in  which  it  is  placed.  Hence,  all  that  is 
necessary  in  order  to  make  concrete  objects  of  any  desired 
shape  or  form  is  to  construct  molds  of  the  proper  shape.  For 
different  objects,  the  forms  vary  in  character  from  simple  two 
by  fours  laid  on  edge,  for  sidewalks,  to  intricate  molds  in 
sand,  clay,  or  plaster  of  Paris,  for  ornamental  objects,  or  to 
the  maze  of  walls,  floors,  columns,  and  braces  in  reinforced 
concrete  buildings.  According  to  the  character  of  the  objects 
being  made  or  the  subsequent  treatment  of  the  surface,  the 
character  of  the  workmanship  required  varies  from  the  simple 
nailing  of  rough  boards  on  studs  to  work  approaching  joinery 
in  its  requirement  of  accuracy  and  skill. 

Materials  Used  for  Forms.  —  Any  material  which  will  hold 
the  concrete  in  place  until  it  has  set  may  be  used  for  a  form. 
Materials  frequently  used  are  earth,  sand,  cast  iron,  sheet  steel, 
and  wood.  For  foundation  and  other  walls  below  ground,  where 
the  earth  will  stand,  the  concrete  is  often  placed  directly  against 
the  earth.  If  this  is  done,  the  excavation  should  be  made  ac- 
curately in  the  proper  place  and  the  walls  should  be  cut  true 
to  surface.  If  the  earth  walls  have  caved  out  to  any  extent,  it 
may  be  more  economical  to  use  wooden  forms  to  save  concrete. 
Moist  sand  is  used  for  molds  for  ornamental  work  of  concrete 
in  much  the  same  manner  in  which  it  is  used  for  molten  metal 
in  the  foundry.  A  wooden  or  other  model  is  used  as  a  pattern 
and  a  liquid  mortar  or  grout  is  poured  into  the  impression 
made  in  the  sand.  Cast-iron  molds  are  often  used  for  objects 
of  small  size  which  are  to  be  made  repeatedly,  such  as  building 
blocks  and  concrete  tile.  Sheet  steel  is  often  used  for  forms  on 
either  small  or  large  work,  where  they  can  be  used  repeatedly, 

62 


CONSTRUCTION  OF  FORMS  63 

as  in  tunnels,  culverts,  fence  posts,  and  silos.  Wood  is,  however, 
the  most  commonly  used  forming  material.  The  ease  with 
which  it  can  be  worked  into  the  desired  form,  and  its  cheap- 
ness, make  it  highly  useful  for  this  purpose. 

Use  of  Wood  for  Forms.  —  The  woods  most  used  for  forms 
are  yellow  pine,  fir,  and  spruce.  A  wood  should  be  chosen 
which  will  not  warp  or  swell  excessively  when  wet.  Green 
lumber  is  better  than  well-seasoned  material  for  this  reason. 
For  exposed  faces  of  concrete  walls,  the  lumber  should  be  free 
from  knotholes,  irregularities,  and  slivers,  and  should  be.  sur- 
faced on  one  side.  The  edges  should  be  surfaced,  or  tongued 
and  grooved.  Six-inch  tongued  and  grooved  flooring  or  eight- 
inch  shiplap  makes  excellent  form  lumber.  Boards  wider  than. 
8  inches  are  difficult  to  draw  together,  if  they  are  crooked, 
and  are  not  recommended  for  general  use.  Either  one-inch 
or  two-inch  lumber  may  be  used  for  face  boards,  and  the 
choice  will  depend  on  the  character  of  the  work.  Where 
a  considerable  amount  of  lumber  is  required,  and  it  can  be 
used  only  once  or  twice,  it  will  be  cheaper  to  use  one-inch 
material,  while  if  the  forms  can  be  used  repeatedly,  two-inch 
stuff  will  be  better.  When  the  face  of  the  work  is  to  be  cov- 
ered, or  when  its  appearance  is  unimportant,  any  rough  lumber 
may  be  used  if  the  cracks  are  battened  to  prevent  leakage  of 
the  mortar. 

The  face  boards  of  forms  should  be  nailed  to  the  studs  or 
joists  as  lightly  as  is  consistent  with  keeping  them  in  place 
until  the  concrete  is  poured.  This  is  especially  important  when 
the  form  is  to  be  torn  apart  after  it  has  been  used  once.  Six- 
penny nails  should  be  used  for  one-inch  forms.  One  nail  to 
each  intermediate  stud  and  two  nails  to  each  end  stud  are 
usually  sufficient.  Heavily  nailed  forms  cannot  be  torn  apart 
without  considerable  injury  to  the  lumber. 

It  is  important  that  no  cracks  or  knotholes  be  left  in  the 
forms  large  enough  to  permit  the  thin  grout  to  flow  out.  This 
grout  contains  the  cement,  and  a  small  crack  will  drain  out 
enough  of  it  to  weaken  the  concrete  near  the  crack  and  make 
it  open  and  porous. 

It  will  help  to  preserve  the  forms  and  prevent  particles  of 


64     CONCRETE  CONSTRUCTION  FOR   RURAL   COMMUNITIES 

concrete  from  sticking  to  them  if  they  are  covered  with  a  coat 
of  crude  oil,  linseed  oil,  or  soap  solution.  When  this  is  not 
done,  they  should  be  thoroughly  wet  down  immediately  before 
the  concrete  is  poured.  The  forms  should  be  thoroughly  cleaned 
of  all  particles  of  concrete  adhering  to  them  before  they  are 
used  a  second  time.  If  this  is  not  done,  the  surface  of  the 
wall  will  be  made  rough  and  irregular.  The  cleaning  can  easily 
be  done  with  a  wire  brush  or  a  stiff  broom,  assisted  by  a  little 
scraping. 

Tying  and  Bracing  Forms.  —  The  tying  and  bracing  of  forms 
is  highly  important.  Those  who  have  not  investigated  the 
matter  cannot  realize  the  tremendous  pressure  exerted  by  the 
concrete,  tending  to  separate  and  displace  the  forms.  The  pres- 
sure, in  pounds,  exerted  on  each  square  foot  of  the  forms  by 
concrete  in  a  semifluid  state  can  be  found  approximately  by 
multiplying  the  height  of  the  concrete  above  the  point  in  ques- 
tion by  150,  since  150  pounds  is  about  the  weight  of  a  cubic 
foot  of  concrete.  For  example,  in  a  wall  five  feet  high  and  of  any 
thickness,  the  pressure  tending  to  separate  the  forms  at  the 
bottom,  while  the  concrete  is  semifluid,  is  about  5  X  150  =  750 
pounds  per  square  foot.  The  forms  must  therefore  be  tied  and 
braced  sufficiently  to  resist  this  pressure.  Most  carpenters 
and  others  who  have  not  had  actual  experience  in  forming  for 
concrete  fail  to  realize  this,  and  many  bulged  walls  are  the 
result.  Where  forms  are  used  on  both  sides  of  the  wall,  the 
ties  should  extend  through  the  wall  from  one  form  to  the  other, 
to  take  up  this  pressure.  The  bracing  to  the  ground  then  re- 
quired is  only  sufficient  to  hold  the  wall  itself  upright,  and  to 
support  the  shock  and  jar  of  placing  the  concrete.  It  is  en- 
tirely unsatisfactory  to  try  to  resist  the  pressure  of  the  concrete 
by  means  of  stakes  and  diagonal  braces  running  to  the  ground. 
If  this  is  attempted  for  walls  more  than  a  foot  or  two  in  height, 
failure  is  almost  sure  to  result. 

Forms  for  Straight  Walls.  —  The  form  for  a  straight  wall  is 
the  most  common  type  required  and  is  one  of  the  most  easily 
constructed.  It  is  usually  made  of  face  boards  running  hori- 
zontally, nailed  to  upright  studding  or  posts.  If  a  long  wall  is 
to  be  constructed,  use  2"X  6"  plank  for  the  face  boards,  nailing 


CONSTRUCTION  OF  FORMS 


65 


Spacer 


these  to  4"X  4"  or  4"X  6"  uprights  spaced  about  three  feet  on 
centers,  with  twelve-penny  nails.  The  forms  may  be  built  con- 
tinuous, or  may  be  made  as  panels,  each  about  twelve  feet 
long  and  as  wide  as  the  wall  is  high.  These  panels  can  be 
made  up  flat  on  the  ground  and  then  raised  and  supported  in 
place.  The  form  for  one  side  of  the  wall  should  be  lined  up 
and  plumbed,  and  then  thoroughly  braced  by  diagonals  running 
from  the  studs  to  stakes  driven  in  the  ground  or  to  some  adja- 
cent structure.  The  form  for  the  opposite  side  of  the  wall 
should  then  be  placed  and  secured  to  the  one  already  set  by 
ties  and  braces.  Sticks  about  1"X  2",  and  of  a  length  exactly 
equal  to  the  thickness  of  wall  desired,  should  be  cut  for  use  as 
spacers  between  the 
forms.  The  ties  may 
be  made  of  No.  10 
smooth  wire  passed 
around  the  studs  on 
opposite  sides  of  the 
walls  in  a  loop  and 
then  twisted  up  by 
means  of  a  rod  or 
stick.  To  prevent 
drawing  the  forms 
too  close  together,  a 
spacer  should  be 
placed  near  each  tie 
when  it  is  being 
twisted  up.  Most  of 
these  spacers  can  be 
removed  immediately 
after  the  adjacent 
wire     has     been 

tightened,  but  a  sufficient  number  should  be  left  in  place  to  hold 
the  forms  apart  until  the  concrete  is  placed.  These  should  be 
knocked  out  and  removed  just  before  they  are  covered  by  the  con- 
crete. The  method  of  tying  and  bracing  is  clearly  shown  in  Fig. 
18.  Bolts  are  sometimes  used  instead  of  wires  for  the  ties,  but 
they  are  more  expensive  and  are  not  so  adaptable  to  different 


Fig.  18.  —  Tying  and  Bracing  of  Straight  Wall 
Forms. 


66     CONCRETE  CONSTRUCTION  FOR  RURAL  COMMUNITIES 

situations.  Cleats  nailed  to  the  studs  may  be  used  as  ties  at 
the  top  of  the  wall  if  desired. 

The  number  of  ties  required  does  not  depend  on  the  thick- 
ness of  the  walls,  since  the  pressure  of  the  concrete  tending  to 
force  the  forms  apart  is  practically  the  same  for  a  wall  6  inches 
thick  as  for  one  3  feet  or  more  thick.  Usually  ties  should  be 
placed  about  3  feet  or  less  apart  on  each  stud  vertically,  but  a 
little  closer  together  near  the  bottom  than  near  the  top  of  the 
wall.  Where  there  will  be  a  greater  depth  of  unset  concrete  at 
any  one  time  than  four  to  six  feet,  the  spacing  of  the  tie  wires 
near  the  bottom  must  be  less  than  the  value  just  given.  It 
should  be  kept  in  mind  that,  when  the  forms  have  once  bulged, 
it  is  practically  impossible  to  straighten  them,  so  that  ample 
tying  or  bracing  must  be  provided  before  work  is  begun  if  the 
job  is  to  be  satisfactory. 

After  the  concrete  becomes  sufficiently  hard,  the  tie  wires 
may  be  cut,  and  the  panels  moved  along  the  wall  to  be  used 
again  for  forms.  The  wires  may  then  be  clipped  close  to  the 
face  of  the  concrete. 

More  commonly  the  forms  for  small  jobs  can  be  used  only 
once  without  being  torn  apart,  and  for  such  work  one-inch 
face  lumber  should  be  used.  The  uprights  may  be  2"X  4" 
sticks  and  should  be  spaced  about  18  inches  apart  on  centers 
along  the  form.  It  will  often  be  desirable  to  build  the  forms 
on  the  ground  in  sections  as  described  for  the  two-inch  panels, 
afterwards  raising  and  bracing  them.  This  is  especially  con- 
venient in  constructing  thin  walls.  The  bracing  and  tying  may 
be  done  as  described  above  for  the  two-inch  panels. 

Forms  for  Circular  Walls.  —  Concrete  is  often  required  in 
the  form  of  cylindrical  walls,  as  in  water  tanks,  silos,  and  cis- 
terns. In  such  cases,  the  forms,  if  of  wood,  must  be  made 
with  the  face  lumber  running  parallel  to  the  axis  of  the  cylin- 
der, this  being  held  in  place  by  circular  ribs,  cross  braced  to 
make  them  keep  their  shape.  The  face  lumber,  or  lagging,  as 
it  is  called,  may  usually  be  six  inches  wide,  but,  for  small 
circles,  four  inches  is  better.  If  wooden  forms  are  used  on  both 
sides  of  the  wall,  ties  may  extend  through  the  wall,  as  in  the 
case  of  straight  forms.     When  the  earth  answers  for  the  outer 


CONSTRUCTION  OF  FORMS 


67 


form,  the  inner  one   may  be  held  against  the  pressure  of    the 
concrete  by  cross  bracing  to  the  opposite  wall. 


Pencil 


Outer  Form  Inner  Form 

Fig.  19.  —  Laying  Out  Ribs  for  Circular  Forms. 

A  simple  method  of  laying  out  the  circular  ribs  is  as  follows: 
Fasten  a  string  to  a  nail  in  the  top  of  a  stake  driven  firmly 
in  the  ground.     Measure  off  a  length  of  string  equal  to  one- 
half  the  diameter  of  the  circle  desired  and  tie  a  knot  through 
which  a  nail  is  forced.     Keeping  the  string  tight,  draw  a  circle 


l""jb  m-i  J~-  v  t»'^g 

1 

1 

1 

ii 

IN 

Forms  in  Place  'Vertical  Section 

Fig.  20.  —  Construction  of  Circular  Forms. 

on  the  ground  with  the  nail.  Lay  the  boards  around  this  circle 
as  shown  in  Fig.  19,  and  nail  them  together  securely.  Then 
mark  out  the  circle  on  them,  and  saw  along  the  line.  Figure 
20  shows  the  form  completed,  with  the  concrete  in  it. 


68    CONCRETE  CONSTRUCTION  FOR  RURAL  COMMUNITIES 

Forms  for  Overhead  Floors  and  Roofs.  —  Overhead  floors  and 
roofs  of  reinforced  concrete  usually  consist  of  floor  panels  or 
slabs  of  comparatively  small  thickness,  resting  on  beams  and 
girders,  the  latter  being  supported  by  the  columns  or  walls. 
The  beams  project  below  the  floor  panels,  the  top  of  the  floor 
being  carried  level  over  the  beams. 


Fig.  21.  —  Reinforced  Concrete  Floor  Forms  Under  Construction. 


In  the  construction  of  forms  for  such  a  floor,  the  forms  for 
the  beams  are  usually  made  first.  These  forms  are  supported 
near  the  ends  and  at  one  or  more  intermediate  points  by  up- 
right timbers  resting  on  planks  or  other  timbers  laid  flat  on 
the  floor  below.  The  forms  for  the  floor  panels  are  made  of 
1"X  6"  boards  on  two-inch  joists.  The  ends  of  these  joists  are 
supported  on  the  sides  of  the  beam  forms.  Usually  it  will  also 
be  necessary  to  put  one  or  more  intermediate  rows  of  supports 
under  the  floor  panel  joists. 

In  putting  the  forms  together,  the  problem  of  taking  them 
down   easily  with  as  little   damage    as   possible  to  the  lumber 


CONSTRUCTION  OF  FORMS 


69 


and  to  the  concrete  should  be  kept  constantly  in  mind.  The 
boards  should  be  nailed  very  lightly  to  the  joists,  and  all  pieces 
of  face  lumber  should  be  cut  just  a  little  short,  so  that  their 
ends  will  not  project  into  the  concrete.  All  supports  should  be 
cut  an  inch  or  two  short,  and  wedges  used  to  bring  them  up  to 
the  proper  place.  This  makes  it  easy  to  level  up  the  floor,  and 
to  remove  the  supports  when  the  forms  are  being  taken  down. 
Figure  21  shows  the  forms  for  a  reinforced  concrete  floor  under 
construction. 


Fig.  22.  —  Column  Forms. 

Column  Forms.  —  Column  forms  may  often  be  used  re- 
peatedly if  properly  designed  and  handled.  The  sides  may  be 
built  in  separate  panels,  these  being  lightly  tacked  together 
and  the  whole  being  securely  clamped  either  by  means  of 
wooden  frames  nailed  around  the  columns  as  shown  in  Fig. 
22,  or  by  means  of  bolts  or  some  kind  of  patented  clamp,  of 
which  there  are  several  on  the  market.  An  opening  should 
be  left   at  the  bottom  of  each   column,  through   which   chips, 


70     CONCRETE  CONSTRUCTION  FOR  RURAL  COMMUNITIES 

sawdust,  and  dirt  may  be  cleaned  out.  This  can  be  closed  up 
just  before  concreting  is  begun  and  after  the  base  of  the  form 
is  carefully  cleaned  out.  The  same  precaution  should  be  ob- 
served of  cutting  the  forms  a  little  short  as  in  the  case  of  the 
floor  forms,  to  make  their  removal  easy. 

Time  of  Removal  of  Forms.  —  The  setting  and  hardening 
of  concrete  is  a  gradual  process,  and  its  rate  is  dependent  on 
several  factors.  (See  "  Setting  of  Concrete,"  in  Chapter  VI.) 
For  this  reason  the  time  which  must  elapse  before  the  forms 
may  be  removed  will  differ  materially  in  different  cases  for 
the  same  kind  of  structure.  Warm  weather,  rich  mixtures,  and 
a  dry  consistency  all  tend  to  make  concrete  set  rapidly  and 
hence  make  it  possible  to  remove  forms  earlier  than  when  these 
conditions  are  not  present. 

Concrete  not  subjected  to  any  load  but  its  own  weight,  and 
in  such  a  form  that  no  tendency  to  bend  is  produced,  will  be 
set  sufficiently  to  permit  the  forms  to  be  removed  when  it  is  so 
hard  that  it  cannot  be  indented  by  pressure  with  the  thumb. 
If  the  concrete  is  loaded,  or  if  there  is  a  bending  tendency,  as 
in  beams  and  floors,  the  forms  must  be  left  much  longer.  In 
reinforced  concrete  work  the  forms  should  usually  remain  in 
place  for  from  one  to  four  weeks.  In  cool  weather  they  should 
be  allowed  to  remain  at  least  twice  as  long  as  in  warm  weather. 
It  is  often  desirable  to  leave  certain  braces  and  supports  in 
place  under  beams  and  slabs  when  the  rest  of  the  forms  are 
removed.  An  experienced  judgment  is  required  to  tell  the 
earliest  date  at  which  many  forms  may  be  removed,  and  the 
novice  should  be  careful  to  allow  plenty  of  time. 


CHAPTER  VI 
MIXING   AND    HANDLING   CONCRETE 

Requirements  of  Good  Mixing.  —  A  concrete  is  well  mixed 
when 

(1)  every  particle  of  sand  and  stone  is  coated  with  cement 
paste, 

(2)  the  sand  and  stone  are  evenly  distributed  through  the 
mass,  and 

(3)  the  whole  mass  is  of  a  uniform  consistency. 

If  a  batch  of  concrete  has  light  streaks  through  it,  or  has 
some  stones  uncoated,  or  if  parts  of  it  have  a  deficiency  or  excess 
of  mortar  or  of  water,  then  the  mixing  has  not  been  sufficiently 
thorough,  and  an  inferior  grade  of  concrete  is  produced. 

Consistency.  —  It  is  impossible  to  give  definite  rules  regard- 
ing the  amount  of  water  to  be  used  in  mixing  concrete,  as 
this  depends  to  a  considerable  extent  on  the  brand  of  cement, 
the  characteristics  of  the  sand  and  stone,  the  proportions  used, 
and  the  consistency  desired  in  the  concrete.  The  amount  of 
water  required  should  be  determined  by  trial  for  the  first  batch 
mixed  on  any  given  job,  and  the  same  amount  should  be  used 
for  subsequent  batches.  All  water  used  should  be  reasonably 
clean,  and  free  from  acid,  alkaline  or  organic  impurities. 

The  proper  consistency  for  concrete  differs  according  to  the 
purpose  for  which  it  is  to  be  used.  Experiment  has  shown 
that  the  strongest  concrete  is  obtained  by  the  addition  of  just 
enough  water  so  that,  by  hard  tamping  in  the  forms,  a  little 
water  may  be  made  to  flush  to  the  surface.  The  difference  in 
strength  is  slight,  however,  and  if  the  concrete  does  not  receive 
sufficient  tamping,  such  a  concrete  will  be  considerably  weaker 
than  a  wetter  mixture,  for  which  less  tamping  is  required. 

The  three  different  consistencies  most  used  are : 

(1)   Wet  mixture,  which  is  thin  and  mushy.     It  will  run  off 

71 


72    CONCRETE  CONSTRUCTION  FOR  RURAL  COMMUNITIES 

the  shovel  unless  handled  quickly,  will  spread  out  level  in  a 
wheelbarrow  after  being  wheeled  a  very  few  feet,  and  will  not 
support  the  weight  of  a  man  when  placed  in  the  forms.  This 
mixture  is  suitable  for  thin  walls  and  for  reinforced  work  where 
there  is  an  intricate  network  of  steel  around  which  the  concrete 
must  run.  But  little  compacting  is  required,  a  little  "joggling" 
with  a  spade  being  sufficient.  Care  must  be  taken  not  to  use 
enough  water  to  cause  separation  of  the  materials  while  the 
concrete  is  being  handled.  Cement  and  sand  mixed  to  this 
consistency  is  much  used  under  the  name  of  grout. 

(2)  Medium  mixture,  which  is  wet  enough  to  be  compacted 
by  spading  or  light  ramming,  but  not  so  wet  that  much  free 
water  will  be  brought  to  the  surface  by  such  compacting.  It 
will  quake  freely  like  jelly  and  will  support  a  man  only  after 
he  has  sunk  into  the  surface  some  distance.  This  is  the  most 
suitable  consistency  for  general  concrete  work.  But  little  labor 
is  required  to  work  the  concrete  into  place  and  compact  it, 
yet  it  is  not  so  wet  that  there  is  danger  of  the  separation  of  the 
materials. 

(3)  Dry  mixture,  which  is  of  the  consistency  of  damp  earth. 
A  handful  of  it  squeezed  together  will  retain  its  shape  after 
the  pressure  is  removed,  but  it  is  wet  enough  so  that  a  thin 
film  of  water  can  be  brought  to  the  surface  by  hard  ramming. 

In  making  dry  mixtures,  great  care  must  be  taken  that  enough 
water  is  used,  as  a  little  deficiency  will  produce  a  permanently 
weak  concrete.  Care  must  also  be  taken  to  see  that  this  mix- 
ture is  thoroughly  rammed.  Tests  show  that  concrete  made  of 
dry  mixtures  may  be  ten  to  twenty  times  as  strong  when  heavily 
rammed  as  when  the  concrete  is  lightly  pushed  into  the  molds. 
Figure  23  shows  the  results  of  such  a  test  made  at  the  Kansas 
State  Agricultural  College. 

This  mixture  is  used  when  it  is  desired  to  remove  the  molds, 
or  forms,  almost  immediately,  as  in  building  blocks,  and  also 
when  the  concrete  must  carry  a  considerable  load  in  a  few 
days.  Concrete  made  of  it  and  well  rammed  will  be  stronger 
than  the  wetter  mixtures  for  several  weeks  after  being  mixed, 
but  the  latter  will  gradually  catch  up,  and,  unless  the  dry  mix- 
tures are  very  well  rammed,   will  eventually   become  stronger. 


MIXING  AND  HANDLING  CONCRETE 


73 


Dry  mixtures  also  give  a  more  porous  concrete  and  hence  are 
unsuitable  for  structures  that  must  be  water-proof. 

On  account  of  the  difficulties  in  the  use  of  dry  mixtures,  it  is 
recommended  that  they  be  avoided  wherever  possible. 


Fig.  23.  —  Effect  of  Tamping  on  the  Strength  of  Concrete. 
These  Cylinders  were  made  of  the  Same  Batch  of  Concrete.     One  was 
Thoroughly  Tamped :  the  Other  was  not.     Note  the  Difference  in  the  Breaking 
Loads. 

Methods  of  Mixing.  —  Two  principal  methods  of  mixing  are  in 
use,  hand  mixing  and  machine  mixing.  By  either  of  these 
methods  excellent  concrete  can  be  obtained,  and  the  choice 
will  usually  be  determined  by  the  size  of  the  job  and  the  readi- 
ness with  which  a  mixer  can  be  obtained.  For  very  small  jobs 
hand  mixing  is  often  preferable  on  account  of  the  labor  re- 
quired to  move  the  machine  to  the  job,  though  in  recent  years 
small  and  readily  portable  mixers  have  been  developed  to  such 
an  extent  that  it  is  profitable  to  use  them  on  much  smaller 
jobs  than  formerly.  On  large  jobs,  machine  mixers  should 
always  be  used  on  account  of  the  saving  of  labor. 

Tools  Required  for  Hand  Mixing.  —  Very  few  tools  are  really 
required  for  mixing  concrete  by  hand.  A  shovel,  a  bucket,  and 
a  place  to  do  the  mixing  where  the  water  will  not  wash  away 


74     CONCRETE  CONSTRUCTION  FOR  RURAL  COMMUNITIES 

the  cement  will  answer.     It  will  be  found  convenient,  however, 
to  have  the  following  list  of  tools,  if  much  work  is  to  be  done: 

Shovels,  No.  3  square  pointed,  one  for  each  man, 

Wheelbarrows,  preferably  at  least  two,  with  sheet-steel  bodies, 

Water  barrel, 

Water  buckets,  two  or  more, 

Rammer.    A  2"  X  4"  stick  may  be  used,  or  better,  a  4"  X  4" 

stick  30  inches  long  with  handles, 
Spade,  to  force  stones  back  from  the  forms, 
Sand  screen  (see  Fig.  13), 
Mixing  platform, 
Measuring  boxes. 

A  suitable  platform  for  use  by  two  men  and  large  enough 
for  a  two-bag  batch  of  concrete  (i.e.,  a  batch  requiring  two 
sacks  of  cement)  is  shown  in  Fig.  24.      It  is  made  from  twenty- 


Fig.  24.  —  Tools  Used  in  Hand  Mixing. 

two  pieces  of  1"  X  6"  tongued  and  grooved  flooring,  each  10  feet 
long,  held  together  by  five  2"  X  4"  cleats  about  9  feet  6  inches 
long.  The  upper  side  of  the  flooring  should  be  surfaced  in  order 
to  make  shoveling  easy.  The  boards  should  be  driven  tightly 
together  to  prevent  leakage  of  the  water  and  cement.  Any 
small  knotholes  may  be  covered  by  cleats  on  the  bottom  side. 
Around  the  upper  side  of  the  board  should  be  nailed  a  2"X 
2"  strip  to  keep  the  grout  from  running  off  the  edges.     For  a 


MIXING  AND  HANDLING  CONCRETE  75 

crew  of  four  men,  a  board  10  feet  by  12  feet,  which  is  large 
enough  for  a  four-bag  batch,  may  be  desirable. 

Measuring  the  Materials.  —  A  measuring  box  should  be  made 
to  hold  the  quantity  of  sand  to  be  used  for  each  sack  of  cement, 
and  if  the  amount  of  stone  used  is  not  just  twice  that  of  sand, 
another  box  should  be  made  to  hold  the  quantity  of  stone  used 
for  each  sack  of  cement.  One  bag  of  cement  may  be  consid- 
ered as  one  cubic  foot;  hence,  for  a  1:2J:5  concrete  the  box 
should  be  made  to  hold  2J  cubic  feet.  If  made  27"  long,  16" 
wide,  and  10"  deep  inside,  it  will  hold  just  this  amount.  If 
preferred,  the  boxes  may  be  made  large  enough  to  hold  mate- 
rials for  a  two-bag  batch. 

The  measuring  box  should  be  made  bottomless.  The  two 
longer  sides  may  be  allowed  to  project  about  six  inches  past 
the  ends  and  may  be  cut  down  to  form  handles,  as  shown  in 
the  illustration. 

When  the  materials  must  be  wheeled  in  barrows  it  is  a  com- 
mon practice  to  measure  into  the  barrows  the  proper  amounts 
of  sand  and  stone,  and  thereafter  to  fill  them  to  the  same  height. 
If  care  is  taken  in  filling  the  barrows,  and  if  a  check  is  made 
by  using  the  boxes  occasionally,  this  method  will  give  good  re- 
sults with  a  saving  in  labor,  but  the  proportions  will  usually 
be  less  exact  than  when  the  boxes  are  used. 

Mixing  the  Materials  by  Hand.  —  The  mixing  board  should  be 
set,  if  possible,  so  that  the  concrete,  after  mixing,  can  be  shoveled 
directly  into  place  without  hauling,  and  the  piles  of  materials 
should  be  so  placed  that  they  can  be  shoveled  directly  on  to 
the  board  without  the  necessity  of  loading  into  barrows.  Often, 
however,  it  will  be  impossible  to  arrange  the  work  so  conve- 
niently. In  such  cases,  it  will  probably  be  desirable  to  place  the 
board  so  that  the  materials  can  be  shoveled  directly  on  to  it 
from  the  piles,  and  so  that  there  will  be  but  a  short  distance 
to  wheel  the  concrete  after  it  is  mixed. 

The  mixing  platform  should  be  blocked  up  so  that  it  is  level 
and  will  not  sag  under  the  weight  of  men  and  materials.  Board 
runways  should  be  built  for  the  wheelbarrow  from  the  mixing 
platform  to  the  place  of  depositing  the  concrete,  and  to  the  piles 
of  sand  and  stone,  as  these  will  very  much  lighten  the  work  of 


76     CONCRETE  CONSTRUCTION  FOR  RURAL   COMMUNITIES 

the  wheelers.  They  are  best  made  of  two-inch  plank  about  12 
inches  wide,  but  one-inch  lumber  can  be  used  where  the  runs 
are  on  even  ground.  Runs  elevated  above  the  ground  should 
be  at  least  two  feet  wide. 

The  mixing  platform  and  runs  having  been  placed  in  posi- 
tion, and  the  measuring  box  being  ready,  the  method  of  mixing 
is  as  follows: 

The  measuring  box  is  placed  on  the  platform  near  one  of  the 
ten-foot  sides  and  is  twice  filled  level  full  of  sand.  The  sand  is 
then  leveled  down  to  a  nearly  uniform  depth  of  three  or  four 
inches.  Two  bags  of  cement  are  spread  as  evenly  as  possible 
over  the  sand.  Two  men  stationed  at  opposite  sides  of  the 
pile  then  begin  to  "turn  it  over"  with  shovels,  beginning  at  the 
edge  of  the  pile  farthest  from  the  side  of  the  platform  and 
gradually  working  through  the  pile.  With  a  little  care  in  empty- 
ing the  shovels,  the  mixing  of  the  materials  may  be  made  much 
more  efficient  than  if  the  materials  are  carelessly  shoveled  over 
into  another  pile.  The  shovel  should  be  turned  completely  over 
in  emptying  it  and  it  should  be  drawn  toward  the  shoveler  with 
a  sweeping  stroke  so  as  to  distribute  the  material  over  consid- 
erable space,  instead  of  depositing  it  all  in  a  heap.  Further, 
the  mixture  should  not  all  be  shoveled  together  into  a  pile,  but 
should  be  distributed  in  a  layer  of  uniform  thickness,  similar 
to  the  one  it  occupied  before  commencing  to  turn  it  over.  Care 
should  be  taken  not  to  leave  a  strip  of  materials  in  the  middle 
of  the  pile,  unturned. 

After  the  mass  has  all  been  turned  over,  the  shovelers  turn 
it  back  again  so  that  it  occupies  the  same  position  on  the 
board  as  at  first.  Usually  a  third  turning  will  be  required  be- 
fore it  is  ready  for  the  addition  of  stone  and  water.  If  a  third 
man  is  available  for  mixing,  he  may  use  a  garden  rake  or  a 
hOe  on  the  pile  upon  which  the  other  mixers  are  shoveling. 
He  should  give  special  attention  to  those  portions  which  are 
least  thoroughly  mixed. 

After  the  mass  has  been  turned  the  third  time,  it  should  be 
of  uniform  color,  with  the  cement  uniformly  distributed  through 
the  sand.  If  it  is  not,  mixing  should  be  continued  until  this 
result  is  obtained.     The  mass  should  now  be  spread  out  to  a 


MIXING  AND  HANDLING  CONCRETE  77 

uniform  depth  ready  for  the  stone.  The  measuring  box  may  be 
placed  directly  on  top  of  the  mixture,  filled  level  full,  and 
emptied.  After  the  proper  amount  of  stone  is  added,  it  should 
be  leveled  down  to  a  uniform  depth  over  the  cement  and  sand 
mixture,  and  the  water  added.  The  water  should  be  measured 
in  buckets,  the  required  amount  being  determined  for  the  first 
batch  by  trial  and  the  same  amount  being  used  for  subsequent 
batches.  Only  about  three-fourths  of  the  water  that  will  be 
required  for  the  batch  should  be  added  at  first,  as  otherwise 
some  of  it  is  likely  to  flow  off  the  pile,  washing  away  the  cement 
with  it. 

The  mass  is  now  turned  again,  as  directed  for  the  cement 
and  sand,  water  being  added  to  the  dry  spots  until  the  required 
amount  of  water  has  been  used,  or  the  desired  consistency  has 
been  obtained.  Three  turnings  of  the  mass  after  the  stone  has 
been  added  will  usually  be  sufficient,  but  if  this  does  not  make 
the  mass  of  the  same  consistency  throughout,  or  if  the  mortar 
is  not  uniformly  distributed  through  the  stone,  the  mixing  should 
be  continued  until  these  results  have  been  accomplished.  The 
concrete  is  now  ready  to  be  shoveled  into  place,  or  into  wheel- 
barrows or  carts  if  it  must  be  hauled. 

Some  concrete  workers  vary  the  method  by  shoveling  the 
cement  and  sand  mixture  on  top  of  the  stone,  instead  of  plac- 
ing the  stone  on  top  of  the  cement  and  sand  mixture,  and 
some  prefer  to  wet  the  sand  and  cement  mixture  before  adding 
the  stone.  It  is  believed,  however,  that  the  method  given 
above  will  be  most  generally  satisfactory.  The  details  of  the 
process  may  be  varied  so  long  as  the  ultimate  object  is  ob- 
tained, —  the  thorough  mixing  of  the  materials  into  a  homo- 
geneous mass. 

Machine  Mixing.  —  Machines  suitable  for  use  on  small  con- 
crete jobs  may  be  divided  into  two  general  classes:  (1)  batch 
mixers,  and  (2)  continuous  mixers.  In  the  former,  the  proper 
quantities  of  the  materials  for  a  batch  of  concrete  are  intro- 
duced into  the  machine,  mixed,  and  discharged,  before  addi- 
tional materials  are  added.  In  the  latter  the  materials  are 
continuously  fed  into  one  end  while  the  concrete  is  discharged 
from  the  other  end. 


78     CONCRETE  CONSTRUCTION   FOR  RURAL   COMMUNITIES 

Continuous  Mixers.  —  A  continuous  mixer  usually  consists  of 
a  trough  or  cylinder,  often  inclined,  in  which  the  concrete  is 
mixed  by  means  of  revolving  blades  or  paddles,  which  continu- 
ously  work   the    materials    toward    the    discharge   end   of  the 


Fig.  25.  —  A  Continuous  Mixer. 

trough,  and  at  the  same  time  mix  them  together.  A  continuous 
mixer  is  usually  driven  by  a  gasoline  engine  which  is  mounted 
with  the  mixer  on  a  four-wheeled  truck,  so  that  it  may  readily  be 
moved  from  place  to  place.  The  materials  may  be  measured 
into  the  upper  end  of  the  trough  automatically,  by  means  of  a 
system  of  cupped  rolls  which  rotate  continuously  beneath  hop- 
pers or  by  other  devices,  or  they  may  be  shoveled  into  the 
trough,  the  proportions  depending  on  the  number  of  shovelfuls 
of  each  material  supplied.  Both  methods  of  measurement  are 
open  to  the  objection  that  they  are  likely  to  be  inaccurate,  the 
first  on  account  of  clogging  of  the  pockets  of  the  revolving  rolls, 
or  uneven  feeding  due  to  the  varying  moisture  in  the  sands, 
and  the  second  on  account  of  unevenness  in  the  size  of  shovelfuls 
taken.     In  either  case  it  is  necessary  to  check  the  proportions 


MIXING  AND  HANDLING  CONCRETE 


79 


at  frequent  intervals  by  actual  measurement,  if  satisfactory  results 
are  to  be  obtained.  The  machines  are  usually  made  so  that  the 
materials  will  be  mixed  dry  in  the  upper  end  of  the  trough  before 
they  have  reached  the  point  where  the  water  is  sprayed  in.  The 
lower  end  of  the  trough  is  often  provided  with  a  hood  in  which 
the  mixed  concrete  may  be  caught  while  the  barrows  or  carts  in 
which  it  is  hauled  away  from  the  machine  are  changed. 


Fig.  26.  —  A  Batch  Mixer  of  the  Revolving  Drum  Type. 

There  are  on  the  market  a  number  of  these  machines  which 
will  give  good  results  if  careful  attention  is  paid  to  their  operation. 
They  are  usually  small  and  easily  moved  from  place  to  place,  and 
are  of  low  first  cost.  They  are  used  mostly  on  small  jobs,  such 
as  sidewalks,  curbs  and  gutters,  cellar  floors,  and  house  founda- 
tions. Certain  machines  of  this  class  are  well  adapted  to  mortar 
mixing  and  some  are  not  provided  with  means  for  feeding  stone, 
being  used  only  for  work  in  which  no  stone  is  required. 

Batch  Mixers.  —  A  batch  mixer  consists  usually  of  a  revolv- 
ing cylinder  or  drum,  inside  of  which  blades  or  vanes  are  pro- 
vided to  cut  and  mix  the  material  as  it  is  rolled  over  by  the 


80    CONCRETE  CONSTRUCTION  FOR  RURAL  COMMUNITIES 

turning  of  the  drum.  The  drum  may  be  driven  either  by  a 
steam  or  gasoline  engine,  or  by  an  electric  motor.  The  large 
machines  are  usually  driven  by  steam  engines,  steam  from  the 
boilers  being  used  to  heat  the  mixing  water  in  cold  weather. 
Most  of  the  small  machines  are  driven  by  gasoline  engines. 

The  engine  and  mixer  may  be  mounted  on  a  four-wheeled 
truck,  or  may  be  mounted  on  skids  if  it  is  not  required  that 
the  machine  be  readily  portable.  These  machines  are  often 
provided  with  a  hopper,  or  skip,  of  size  sufficient  to  hold  one 
batch  of  materials,  with  a  hoisting  rig,  so  that  this  may  be  ele- 
vated from  the  ground  for  charging  the  mixer.  The  skip  may 
then  be  filled  again  while  the  previous  batch  is  being  mixed. 
Batch  mixers  are  often  provided  with  water  drums  with  auto- 
matic means  for  measuring  the  same  amount  of  water  for  each 
batch.  In  the  larger  machines  the  materials  are  usually  intro- 
duced at  one  end  of  the  drum,  and  after  these  are  mixed  the 
concrete  is  drawn  out  of  the  other  end,  either  partly  or  com- 
pletely, before  the  next  batch  is  introduced. 

Several  small  machines  have  been  recently  brought  out  which 
employ  a  drum  closed  at  one  end,  the  materials  being  introduced 
into  the  open  end  on  one  side  of  the  machine.  After  these  are 
mixed,  the  drum  is  tipped  over  so  as  to  discharge  on  the  op- 
posite side.  This  results  in  simplifying  the  machine  and  cut- 
ting down  the  first  cost  to  a  point  where  it  can  compete  with 
continuous  mixers  for  small  jobs,  while  maintaining  the  advan- 
tages of  batch  mixing.  The  author  believes  that  this  type  of 
machine  will  grow  in  favor  and  will  to  a  considerable  extent 
displace  the  continuous  mixer  for  small  work. 

The  advantages  of  the  batch  mixers  are  that  the  proportions 
can  be  accurately  measured  and  the  materials  can  be  mixed 
as  long  and  as  thoroughly  as  is  desired.  The  measurement  of 
materials  is  usually  by  wheelbarrows,  filled  to  the  proper  height, 
as  was  discussed  in  the  treatment  of  hand  mixing. 

A  very  common  size  of  batch  is  one  requiring  one  bag  of 
cement,  commonly  known  as  a  one-bag  batch.  This  is  a  con- 
venient size,  as  it  does  not  require  the  division  of  the  packages 
of  cement,  the  bags  of  the  latter  being  emptied  directly  into  the 
mixer  or  into  the  loading  skip.     Larger  machines  use  two-bag 


MIXING  AND  HANDLING  CONCRETE 


81 


batches.  A  convenient  method  of  measurement  for  smaller  ma- 
chines is  by  the  use  of  14-quart  buckets,  the  cement  being  first 
emptied  into  a  box,  from  which  it  can  be  conveniently  dipped  by 
the  bucket.  Care  should  be  taken  to  see  that  the  buckets  are 
filled  to  the  same  height  with  the  cement  as  with  the  sand  and 
stone,  and  that  the  buckets  of  cement  are  lightly  packed  or  shaken, 
while  the  buckets  of  sand  and  stone  are  loosely  filled. 


Fig.  27.  —  A  Small  Batch  Mixer  of  the  Revolving  Tub  Type. 


Transporting  Concrete.  —  On  small  jobs  the  concrete  can 
sometimes  be  shoveled  directly  into  place,  from  the  mixing 
board;  or,  if  a  machine  is  used  for  mixing,  it  can  sometimes 
be  so  located  that  the  concrete  can  be  dumped,  or  spouted 
directly  into  place.  More  often  it  will  be  necessary  for  the 
concrete  to  be  hauled  from  the  mixing  platform  or  the  mixer 
to  the  forms.  Steel  wheelbarrows  or  steel  carts  are  usually 
used  for  this  purpose.  For  the  small  contractor  or  concrete 
user,  the  barrows  are  preferable  on  account  of  their  being  more 
generally  serviceable.  For  thin,  high  walls  and  columns,  gal- 
vanized buckets  will  be  found  useful. 


82    CONCRETE  CONSTRUCTION  FOR  RURAL  COMMUNITIES 

On  jobs  of  considerable  magnitude,  towers  are  sometimes 
built  in  some  central  place,  in  which  the  concrete  is  elevated  to 
a  considerable  height  above  the  work.  It  is  there  dumped  into 
a  hopper,  from  which  it  is  spouted  to  the  various  parts  of  the 
work.  The  advantage  of  this  method  is  that  the  work  of  trans- 
porting the  concrete  is  all  done  at  one  place,  the  tower,  so 
that  a  hoisting  engine  or  a  team  can  be  used,  instead  of  the 
expensive  hand  labor  of  wheeling.  On  jobs  below  ground  level, 
such  as  cellar  floors,  the  method  of  spouting  the  concrete  into 
place  can  be  used,  but  without  the  necessity  of  first  hoisting  it. 

Care  should  be  taken  to  see  that  the  materials  of  the  con- 
crete do  not  separate  while  being  wheeled  or  spouted  into 
place.  Separation  is  most  likely  to  occur  if  the  materials  are 
too  wet  or  too  dry.  When  mixed  to  a  medium  consistency, 
concrete  can  be  wheeled  or  spouted  long  distances  with  little 
trouble  from  this  cause.  The  spouts  should  preferably  be 
only  steep  enough  to  permit  the  concrete  to  run  freely.  If  it 
•is  necessary  that  they  be  steep,  they  should  be  entirely  en- 
closed to  avoid  waste  of  materials  and  to  assist  in  preventing 
separation  of  the  stone  from  the  mortar.  In  case  separation 
should  occur,  the  materials  should  be  thoroughly  remixed  be- 
fore the  concrete  is  allowed  to  harden.  A  limited  amount  of 
remixing  can  be  done  in  the  forms  by  thorough  spading. 

Depositing  Concrete.  —  All  concrete  should  be  placed  in  the 
forms  promptly  after  being  mixed.  Any  handling  or  working 
after  it  begins  to  stiffen  will  result  in  a  loss  of  strength.  The 
practice  of  retempering  concrete,  i.e.,  remixing  it  with  water 
after  it  has  begun  to  set,  is  to  be  discouraged. 

All  concrete  must  be  carefully  and  thoroughly  worked  into 
place  and  compacted  to  get  the  best  results.  If  a  rather  dry 
mixture  is  used,  it  should  be  placed  in  layers  about  six  inches 
thick  and  heavily  rammed  until  all  air  has  been  expelled  and 
some  water  has  flushed  to  the  surface.  With  a  somewhat  wet- 
ter mixture,  lighter  ramming  will  be  sufficient.  A  satisfactory 
rammer  for  this  purpose  can  be  made  by  nailing  two  1"X  3" 
pieces  4  feet  long  for  handles,  on  a  4"X  4"  stick  about  30  inches 
long,  or  a  2"X  4"  stick  about  6  feet  long  will  do.  A  spade  or  a 
sharp-edged  stick  should  be  used  to  force  the  stones  away  from 


MIXING  AND  HANDLING  CONCRETE 


83 


the  forms  and  allow  mortar  to  flow  in,  as  otherwise  air  pockets 
will  be  left  next  to  the  forms,  and  when  the  latter  are  removed 
the  surface  of  the  concrete  will  present  a  "  popcorn  effect. "  If 
the  mixture  is  made  wet,  or  if  the  mass  is  fairly  heavy,  a 
spade  will  be  satisfactory  for  compacting  it.  The  spade  should 
be  worked  through  the  concrete  to  expel  all  air  and  to  work 
each  successive  batch  down  into  that  previously  placed. 

Concrete  can  be  deposited  in  water  with  good  results  if 
proper  precautions  are  taken.  It  will  harden  and  become  as 
strong  in  water  as  in  air.  It  is  only  necessary 
to  get  it  into  place  in  some  manner  such  that 
the  cement  will  not  be  washed  out  by  the  water. 
Perhaps  as  satisfactory  a  method  as  any  is  to 
pass  it,  in  a  continuous  flow,  through  a  sheet- 
metal  tube  about  eight  inches  in  diameter.  The 
concrete  within  the  tube  should  be  kept  con- 
stantly at  a  higher  level  than  that  of  the  water 
outside.  This  can  easily  be  accomplished  by 
allowing  the  lower  end  of  the  tube  to  rest  on  the 
mass  which  has  been  deposited,,  and  raising  it 
only  as  is  necessary  to  allow  sufficient  concrete 
to  flow  out.  Forms  or  cofferdams  will  usually  be 
required  to  keep  the  concrete  in  place  and  pre- 
vent the  cement  from  being  washed  away.  These 
need  not  be  water-tight,  however. 

Curing  of  Concrete.  —  If  freshly  poured  con- 
crete is  exposed  to  the  direct  action  of  the  sun 
and  wind,  it  may  dry  out  so  rapidly  as  to  weaken 
it.  In  hot,  dry  weather  it  should  be  shaded  from 
the  sun  for  the  first  week  or  ten  days  by  allow- 
ing the  forms  to  remain  in  place,  or  by  covering 
it  with  canvas,  burlap,  or  sand.  If  this  covering 
is  kept  wet,  the  conditions  for  curing  will  be  ideal. 
It  is  especially  important  that  dry  mixtures  be  C°ncrete  under 
kept  sprinkled. 

Green  concrete  should  be  protected  from  excessive  loads  or  blows 
which  might  result  in  injury.  Light  loads  which  do  not  cause  vi- 
bration may  be  applied,  if  the  resulting  stress  is  pure  compression, 


Fig.28.  —  Tube 
for  Depositing 


84     CONCRETE  CONSTRUCTION  FOR  RURAL  COMMUNITIES 

when  the  concrete  is  but  a  few  days  old.     When  the  loads  pro- 
duce bending  stresses,  a  much  longer  period   should  be  allowed. 

Bonding  Old  and  New  Concrete.  —  Fresh  concrete  will  not 
bond  readily  to  concrete  which  has  set.  In  warm  weather  even 
an  hour  is  sufficient  time  for  the  concrete  to  stiffen  up  suffi- 
ciently so  that  fresh  concrete  will  not  bond  to  it  readily,  espe- 
cially if  a  scum  has  formed  over  the  surface.  This  results  in  a 
seam  being  formed  at  this  section,  which  is  particularly  objec- 
tionable in  structures  which  must  be  water-tight,  because  liquid 
will  seep  through  at  the  seam.  To  avoid  this  trouble,  the 
surface  of  the  concrete  should  be  left  rough,  when  work  is 
stopped;  and  when  it  is  begun  again,  the  scum  should  be  care- 
fully cleaned  off.  It  will  often  be  desirable,  especially  if  a  water- 
tight bond  is  desired,  to  tear  up  the  old  surface  with  a  pick  so 
as  to  expose  the  aggregate.  A  wash  of  neat  cement  and  water, 
or  a  rich  mortar  of  about  1:1,  mixed  to  a  creamy  consistency, 
should  be  applied  to  the  surface  of  the  old  concrete  just  before 
concreting  is  resumed.  If  the  old  concrete  is  dried  out  before 
the  new  is  to  be  placed,  it  should  be  thoroughly  soaked  up  with 
water  before  the  cement  wash  is  applied.  Steel  rods  embedded 
for  half  their  depth  in  the  old  concrete  assist  in  bonding  the  old 
and  the  new  work,  and  in  heavy  walls  large  stones  are  some- 
times embedded  for  half  their  depth  before  work  is  stopped, 
the  projecting  part  acting  as  a  dowel  to  tie  the  new  and  old 
work  together  when  work  is  resumed. 

Contraction  and  Expansion  Joints.  —  Concrete,  like  all  other 
materials,  contracts  and  expands  with  changes  of  temperature. 
The  amount  of  this  change  is  practically  the  same  as  for  steel 
with  equal  temperature  differences.  This  is  very  fortunate,  as 
it  permits  these  two  materials  to  be  used  together  in  reinforced 
concrete  work  without  causing  high  stresses.  The  coefficient 
of  expansion  of  concrete  is  about  0.000,006,  which  means  that 
for  each  degree  change  of  temperature,  a  piece  of  concrete  will 
change  in  length  0.000,006  inches  for  each  inch  it  is  long.  Thus 
the  change  in  length  of  a  concrete  sidewalk  200  feet  long,  when 
the  temperature  changes  from  100  degrees  in  the  summer  to 
0°  in  the  winter,  is 

100  X  200  X  12  X  0.000,006  =  1.44  inches, 


MIXING  AND  HANDLING  CONCRETE  85 

or  nearly  an  inch  and  a  half.  As  friction  prevents  the  sidewalk 
from  slipping  over  the  ground,  the  walk  must  crack,  unless 
cracks  have  already  been  provided.  For  this  reason  it  is  de- 
sirable to  provide  contraction  joints  in  such  cases,  so  that  the 
cracks  may  be  straight  and  uniform  instead  of  being  crooked 
and  irregular.  These  joints  should  be  placed  at  such  intervals 
that  the  contraction  due  to  temperature  change  will  not  cause 
the  walk  to  crack  between  them.  The  joints  should  pass 
entirely  through  the  walk,  not  merely  through  the  wearing 
surface,  as  otherwise  when  the  crack  comes  it  may  not  follow 
the  joint.  For  sidewalks,  the  blocks  should  not  be  much  larger 
than  five  feet  square.  Objects,  such  as  cellar  floors,  which  are 
not  subject  to  extreme  changes  of  temperature,  may  be  laid  in 
larger  blocks  without  danger  of  cracking. 

In  long  stretches  of  concrete  it  is  desirable  to  allow  expan- 
sion joints  as  well  as  contraction  joints,  to  permit  of  the  expan- 
sion of  the  concrete  in  hot  weather.  These  differ  from  the 
contraction  joints  in  that  a  small  space  is  left  between  adjacent 
sections  of  concrete.  To  keep  this  space  from  becoming  filled 
with  material  which  will  prevent  the  expansion  of  the  con- 
crete, it  is  often  filled  with  some  substance,  such  as  tar,  which 
can  easily  be  squeezed  out  in  hot  weather  when  the  expansion 
occurs.  Expansion  joints  may  be  placed  much  further  apart  than 
contraction  joints,  because  the  compressive  strength  of  concrete  is 
much  greater  than  its  tensile  strength.  If  a  small  space  is  al- 
lowed at  each  contraction  joint,  this  will  take  care  of  expansion; 
otherwise  spaces  about  one-fourth  of  an  inch  wide  should  be  left 
at  intervals  of  about  fifty  feet,  in  sidewalks  and  similar  work. 

Setting  of  Concrete.  —  The  setting  of  concrete  is  different 
from  that  of  lime  mortar  in  several  respects.  The  early  strength 
of  the  latter  is  due  to  the  drying  out  of  the  mortar.  This 
dried  mortar  then  slowly  absorbs  carbon  dioxide  from  the  air 
and  is  partly  changed  to  calcium  carbonate.  The  process  goes 
on  most  rapidly  at  the  surface  and  very  slowly  in  the  interior. 
The  setting  of  concrete,  however,  is  not  at  all  a  drying  process, 
but  a  chemical  change.  This  can  easily  be  seen  from  the  fact 
that  concrete  will  set  and  harden  under  water  as  readily  as  in 
air.     Furthermore,   concrete   contains  within  itself  all   the  sub- 


86     CONCRETE  CONSTRUCTION  FOR  RURAL  COMMUNITIES 

stances  necessary  for  its  hardening,  and  it  is  unnecessary  for 
it  to  absorb  anything  from  the  air,  as  in  the  case  of  lime  mortar. 
The  hardening,  therefore,  goes  on  at  about  the  same  rate  all 
through  the  mass,  however  large. 

It  is  impossible  to  say  just  how  long  it  takes  concrete  to 
gain  its  ultimate  strength.  It  is  known,  however,  that  it  may 
continue  to  get  stronger  for  many  years.  The  rate  of  growth 
in  strength  will  differ  with  the  materials,  the  proportions,  and 
the  consistency  of  the  concrete,  and  with  the  temperature. 
As  a  rough  approximation,  it  may  be  said  that,  under  average 
conditions,  the  compressive  strength-  at  the  end  of  the  first 
month  will  be  about  one-third  more  than  at  the  end  of  the 
first  week,  and  at  the  end  of  six  months  it  will  be  about  one-third 
more  than  at  the  end  of  one  month.  After  this  the  increase  in 
strength  continues,  but  at  a  slower  rate,  the  strength  ultimately 
becoming  about  one-half  more  than  at  the  end  of  one  month. 

Since  setting  and  hardening  of  concrete  are  due  to  chemi- 
cal changes,  the  rate  at  which  they  occur  is  dependent  to  a 
considerable  extent  on  the  temperature.  At  temperatures  near 
freezing  setting  and  hardening  go  on  very  slowly,  while  at  high 
temperatures  the  rate  is  very  rapid.  Manufacturers  of  concrete 
building  blocks,  bricks,  tile,  and  other  articles  often  place 
their  products  in  a  closed  room,  heated  to  a  high  temperature 
and  kept  moist  by  the  use  of  steam,  in  order  to  accelerate  the 
hardening.  In  this  way  the  concrete  will  attain  as  much  strength 
in  twenty-four  hours  as  in  several  days  at  ordinary  temperatures. 

The  effect  of  temperature  on  the  setting  of  concrete  has 
an  important  bearing  on  the  time  of  removal  of  forms.  In 
cold  weather,  especially  in  freezing  weather,  the  forms  must 
be  left  in  place  much  longer  than  in  warm  weather,  and  con- 
crete structures,  especially  reinforced  ones,  must  not  be  loaded  so 
early  in  the  former  case  as  they  may  in  the  latter.  Failure  to  ap- 
preciate this  fact  has  caused  the  collapse,  when  the  forms  were 
removed,  of  a  number  of  concrete  structures  built  in  cold  weather. 

Strength  of  Concrete.  —  Tests  of  the  compressive  strength  of 
concrete  are  now  usually  made  on  molded  cylinders,  8"  in  diam- 
eter by  16"  long,  though  many  tests  have  been  made  on  cubes 
from  8"  to  12"  on  a  side.     The  cylinders  are  to  be  preferred,  as 


MIXING  AND  HANDLING  CONCRETE  87 

the  conditions   of   stress  are  more    like   those   usually  met   in 
concrete   structures.     The   cylinders   will   usually  show   a   unit 


Fig,  29.  —  Testing  a  Concrete  Cylinder. 

strength  of  about  0.73  times  as  much  as  is  shown  by  the  12- 
inch  cubes.     Figure  29  shows  an  8"  X  16"  concrete  cylinder  being 


Cylinder 
Before    Tesii 

iq    1 

1     '     Mix  1 
H  Breaking  L 

.oad-  I93.SX 

Mix  1:2:4 
■  Breaking  Load- 146 JOO'B 

Mix  l.'2j:3 
Breakhj  Load  -106.300 

1         Mix  1.3.6 
■Breaking  Load-  dfl.400  H 

Wm               fBS 

i            w 

"                        ^™ 

HH^HHH 

^IHHI 

Fig.  30.  —  Results  of  Tests  to  Illustrate  the  Effect  of  the  Richness  of  the 
Mixture  on  the  Strength  of  Concrete. 

tested  in  the  laboratory  of  the  Kansas  State  Agricultural  Col- 
lege.    The  machine  shown  can  apply  and  weigh  any  load  up 


88    CONCRETE   CONSTRUCTION  FOR  RURAL  COMMUNITIES 

to  200,000  lb.     This  is  sufficient  to  develop  a  stress   of  about 
4000  lb.  per  square  inch  on  an  8-inch  cylinder. 

The  breaking  strengths  that  may  properly  be  expected  of 
standard  concrete  cylinders  made  of  good  materials  properly 
handled  are  indicated  in  Table  V,  which  gives  maximum  values 
to  be  used  as  the  basis  for  design,  as  recommended  by  the 
Joint  Committee  from  the  national  engineering  societies.  Indi- 
vidual tests  may  show  values  considerably  in  excess  of  those 
indicated,  when  conditions  are  favorable,  but  it  would  not  be 
safe  to  count  on  the  higher  values. 

Table  V 

Crushing  Strength  of  Different  Mixtures  of  Concrete 

(In  pounds  per  square  inch,  at  28  days) 


Aggregate 

Granite,  trap  rock 

Gravel,  hard  limestone  and 
hard  sandstone 

Soft  limestone  and  sandstone. 

Cinders 


1:1:2 

l:lf:3 

1:2:4 

1:2*:  5 

3300 

2800 

2200 

1800 

3000 

2500 

2000 

1600 

2200 

1800 

1500 

1200 

800 

700 

600 

500 

1:3:6 
1400 

1300 

1000 

400 


The  safe  working  values  to  be  used  for  the  strength  of  con- 
crete may  be  obtained  from  the  table  by  dividing  the  amounts 
given  by  the  proper  factor  of  safety. 

In  its  resistance  to  tensile  and  bending  stresses  concrete  is 
relatively  very  weak,  and  it  should  not  be  used  where  it  is 
subjected  to  severe  stresses  of  this  kind  without  being  rein- 
forced with  steel.  In  pure  tension,  the  strength  which  may  be 
expected  of  good  concrete  at  the  end  of  one  month  will  be  about 
as  follows: 


1:2:4  concrete  . 
1:3:6  concrete  . 


150-175  lb.  per  sq.  in. 
100-125  lb.  persq.  in. 


Comparing  these  values  with  those  given  for  the  compres- 
sive strength  above,  we  see  that  concrete  is  only  from  one-tenth 
to  one-fifteenth  as  strong  in  tension  as  in  compression. 


MIXING  AND  HANDLING  CONCRETE  89 

The  strength  of  concrete  in  which  clean  gravel  is  used  as  the 
coarse  aggregate  will  usually  be  a  little  less  than  that  made  in 
the  same  proportions  with  a  good  grade  of  broken  stone,  though 
the  difference  is  not  great  and  tends  to  become  less  as  the  con- 
crete grows  older.  A  thin  film  of  dirt  on  the  gravel  will  greatly 
reduce  the  strength  of  the  concrete. 

The  hardness  and  strength  of  the  stone  has  an  important 
effect  on  the  ultimate  strength  of  concrete.  In  tests  of  speci- 
mens which  have  had  time  to  become  thoroughly  hardened, 
most  of  the  stones  break  off  at  the  planes  of  fracture  instead 
of  pulling  out.  The  resistance  to  failure  would  therefore  evi- 
dently be  increased  by  using  a  stronger  stone.  Trap  rock  and 
granite  make  very  strong  concretes,  hard  sandstones  and  lime- 
stones give  somewhat  lower  strengths,  while  soft  limestones, 
sandstones,  shales,  and  cinders  make  weak  concrete. 

The  strength  of  concrete  which  has  thoroughly  hardened  is 
not  materially  affected  by  weak  acids  such  as  are  found  in  sew- 
age and  silage.  Neither  has  manure  nor  animal  nor  vegetable 
oil  any  material  effect  on  concrete  after  it  has  thoroughly 
hardened.  It  should  be  kept  free  from  these  substances,  how- 
ever, while  it  is  green.  Sea  water  and  the  alkaline  waters  of 
some  of  the  semiarid  regions  have  a  disintegrating  effect  on 
concrete  and  in  time  will  cause  its  destruction.  The  best 
method  of  retarding  this  action  is  by  making  the  concrete 
impervious  to  water  by  using  a  rich  mixture,  and,  perhaps,  by 
the  use  of  waterproofing. 

Effect  of  Freezing  on  Concrete.  —  Natural  cement  concretes 
and  mortars  are  very  seriously  affected  by  freezing  before  they 
have  set.  Alternate  freezing  and  thawing  will  cause  almost 
complete  disintegration. 

Portland  cement  mortars  and  concretes  are  much  less  affected. 
It  is  a  disputed  question  as  to  how  much  injury  is  done  to  them 
by  freezing,  but  it  is  probable  that  if  proper  precautions  are 
taken,  the  injury  is  practically  confined  to  the  surface  of  the 
concrete,  this  injury  often  being  manifested  by  a  scaling  off  of 
a  thin  crust  from  the  surface.  It  should  be  remembered, 
however,  that  the  setting  of  the  concrete  is  very  slow  in  cold 
weather,  and  that  little,  if  any,  setting  can  take  place  when 


90     CONCRETE  CONSTRUCTION   FOR  RURAL   COMMUNITIES 

the  concrete  is  frozen.  Forms  should  therefore  be  left  in  place, 
and  concrete  should  not  be  heavily  loaded  in  cold  weather  until 
it  has  been  given  plenty  of  time  to  harden.  If  the  concrete  is 
kept  from  freezing  until  it  has  once  set,  it  may  be  subjected 
to  very  low  temperatures  without  injury.  The  rate  of  gain  in 
strength  will  be  lessened,  but  the  concrete  will  ultimately  get 
fully  as  strong  as  if  maintained  at  normal  temperatures. 

As  a  general  rule,  it  may  be  said  that  concreting  in  freezing 
weather  should  be  avoided.  However,  if  the  extra  expense 
and  care  are  justified,  the  work  may  be  done  successfully  in 
temperatures  as  much  as  20  degrees  or  more  below  freezing. 
Foundations  and  heavy  walls,  the  face  appearance  of  which  is 
unimportant,  may  be  laid  without  special  precautions  other  than 
to  see  that  the  materials  are  warm  enough  when  mixed  so  that 
the  water  will  not  freeze  in  a  film  on  the  aggregate  or  cement 
particles.  This  would  prevent  the  adhesion  of  the  cement  to 
the  surface  and  would  of  course  cause  failure.  All  frozen  dirt 
and  scum  must  be  broken  off  the  surface  of  the  frozen  concrete 
before  fresh  concrete  is  laid.  Because  of  its  resemblance  to  con- 
crete, frozen  dirt  may  easily  escape  notice  unless  a  careful  ex- 
amination is  made. 

For  structures  which  must  not  be  permitted  to  freeze,  the 
temperature  must  be  artificially  maintained  above  the  freez- 
ing point,  or  the  freezing  point  of  the  concrete  must  be  arti- 
ficially lowered.  The  former  can  be  accomplished  by  enclosing 
the  structure  by  means  of  canvas  or  otherwise,  and  by  the 
use  of  stoves.  If  the  temperature  is  only  a  few  degrees  below 
the  freezing  point,  heating  the  materials  may  enable  the  con- 
crete to  get  its  set  before  it  freezes. 

The  freezing  point  of  the  concrete  can  be  artificially  lowered 
by  the  addition  of  common  salt.  This  is  most  conveniently 
added  to  the  water.  An  amount  of  salt  up  to  10  per  cent  of 
the  weight  of  the  water  may  be  used  without  injury  to  the 
ultimate  strength,  though  the  strength  at  short  periods  is  re- 
duced; but  it  is  not  necessary  to  use  so  much  except  in  very 
cold  weather. 

As  one  cannot  tell  in  advance  how  cold  it  will  get  before  the 
concrete  has  set,  an  arbitrary  proportion  of  salt  may  be  used. 


MIXING  AND  HANDLING  CONCRETE  91 

About  half  a  pound  of  salt  to  each  gallon  of  water  used,  cor- 
responding to  about  two  pounds  for  each  bag  of  cement,  will 
be  sufficient  for  temperatures  several  degrees  below  freezing. 
To  assist  in  retaining  the  heat  of  the  mass,  cement  sacks,  can- 
vas, or  straw  should  be  thrown  over  the  work  when  concreting 
is  finished. 


PART  III 
REINFORCED    CONCRETE 


CHAPTER  VII 
GENERAL   PRINCIPLES 

Necessity  for  Reinforcing.  —  While  concrete  is  strong  in  com- 
pression, it  is  weak  in  tension  and  is  brittle.  In  order  to  render 
it  capable  of  resisting  bending  stresses,  steel  rods  are  often  em- 
bedded in  it.  This  produces  what  is  known  as  reinforced  con- 
crete. The  steel  reinforces,  or  strengthens,  the  concrete  by 
resisting  the  tensile  stress,  or  pull,  while  the  concrete  resists 
the  compressive  stress,  or  push.  In  this  way  concrete  beams 
can  be  made  strong  enough  to  carry  a  load  many  times  as 
heavy  as  they  could  carry  if  not  reinforced,  and  concrete  can 
be  used  in  many  places  in  which  it  would  otherwise  be  wholly 
unsuitable. 

Materials  Used  for  Reinforcement.  —  Steel  is  the  only  mate- 
rial in  common  use  for  reinforcing  concrete,  though  wrought 
iron  may  be  used  to  a  limited  extent.  Much  of  the  material 
often  called  iron  is  really  steel,  there  being  comparatively  little 
of  the  former  material  in  use  for  any  purpose  at  the  present 
time.  Steel  is  stronger  and  cheaper  than  iron,  and  consequently 
for  most  purposes  it  has  very  largely  replaced  the  latter. 

Grades  of  Steel  Used.  —  The  steel  usually  carried  in  stock 
in  blacksmith  shops  and  country  hardware  stores  is  too  soft 
and  weak  to  be  an  economical  reinforcing  material,  though  it  is 
used  to  some  extent.  Two  grades  of  steel  are  commonly  used  for 
reinforcing,  known  respectively  as  medium  steel  and  high  carbon 
steel.  High  carbon  steel  is  stronger  than  medium  steel,  but  is 
more  brittle.  Engineers  are  divided  in  their  opinions  of  the 
relative  merits  of  these  two  grades,  but  the  use  of  high  carbon 
steel  is  increasing.  Either  grade  may  be  used  with  confidence 
if  it  will  meet  the  requirements  of  the  Standard  Specifications 
of  the  American  Society  for  Testing  Materials.  The  physical 
properties  and  tests  therein  specified  are  as  follows: 

95 


96     CONCRETE  CONSTRUCTION  FOR  RURAL   COMMUNITIES 

EXTRACT    FROM    STANDARD    SPECIFICATIONS 
FOR    BILLET-STEEL    CONCRETE    REINFORCEMENT    BARS 

(Adopted  by  the  American  Society  for  Testing  Materials,  1914) 

III.    Physical  Properties  and  Tests 

8.  Tension  Tests.  —  (a)  The  bars  shall  conform  to  the  following 
requirements  as  to  tensile  properties.     (See  table  on  opposite  page.) 

(b)  The  yield  point  shall  be  determined  by  the  drop  of  the  beam 
of  the  testing  machine. 

9.  Modifications  in  Elongation.  —  (a)  For  plain  and  deformed  bars 
over  f  in.  in  thickness  or  diameter,  a  deduction  of  1  from  the  per- 
centages of  elongation  specified  in  Section  8  (a)  shall  be  made  for  each 
increase  of  ^  in.  in  thickness  or  diameter  above  f  in. 

(b)  For  plain  and  deformed  bars  under  jq  in.  in  thickness  or  diame- 
ter, a  deduction  of  1  from  the  percentages  of  elongation  specified  in 
Section  8  (a)  shall  be  made  for  each  decrease  of  y1^  in.  in  thickness  or 
diameter  below  ^7g  in. 

10.  Bend  Tests.  —  The  test  specimen  shall  bend  cold  around  a  pin 
without  cracking  on  the  outside  of  the  bent  portion,  as  follows: 


Table  VI 
Bend  Test  Requirements 


Thickness 

or   diameter 

of  bar 

Plain  bars 

Deformed  bars 

Cold- 
twisted 
bars 

Structural 
steel 
grade 

Inter- 
mediate 
grade 

Hard 
grade 

Structural 
steel 
grade 

Inter- 
mediate 
grade 

Hard 
grade 

Under  f 
inch . . . 

|  inch  or 
over . . . 

180° 
d  =  t 

180° 
d-t 

180° 
d-2t 

90° 

d  =  2t 

180° 
d  =  3t 

90° 
d  =  3t 

180° 

d-t 

180° 
d-2t 

180° 

d«3t 

90° 
d  =  3t 

180° 

d  =  4t 

90° 
d  =  4t 

180° 
d  =  2t 

180° 
d  =  3t 

Explanatory  Note.  —  d  =  the  diameter  of  pin  about  which  specimen  is  bent; 
t  =  the  thickness  or  diameter  of  the  specimen. 

11.  Test  Specimens.  —  (a)  Tension  and  bend  test  specimens  for 
plain  and  deformed  bars  shall  be  taken  from  the  finished  bars,  and 
shall  be  of  the  full  thickness  or  diameter  of  bars  as  rolled;  except  that 
the  specimens  for  deformed  bars  may  be  machined  for  a  length  of  at 
least  9  in.,  if  deemed  necessary  by  the  manufacturer  to  obtain  uniform 
cross-section. 


GENERAL  PRINCIPLES 


97 


t-H       3 


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o3t3 


03 
- 
bD 


w 


03-73 
J3  o3 
3  C 


73 


I 


73 


808 

iO       o 


1>   m 


S*3 

'^3  a* 

d  « 

b£^ 

c  a) 
o  a 


98     CONCRETE   CONSTRUCTION  FOR  RURAL  COMMUNITIES 

(b)  Tension  and  bend  test  specimens  for  cold-twisted  bars  shall  be 
taken  from  the  finished  bars,  without  further  treatment,  except  as 
specified  in  Section  2  (b).1 

12.  Number  of  Tests.  —  (a)  One  tension  and  one  bend  test  shall  be 
made  from  each  melt  of  open-hearth  steel,  and  from  each  melt,  or  lot 
of  ten  tons,  of  Bessemer  steel;  except  that  if  material  from  one  melt 
differs  f  in.  or  more  in  thickness  or  diameter,  one  tension  and  one 
bend  test  shall  be  made  from  both  the  thickest  and  the  thinnest  ma- 
terial rolled. 

(b)  If  any  test  specimen  shows  defective  machining  or  develops 
flaws,  it  may  be  discarded  and  another  specimen  substituted. 

(c)  If  the  percentage  of  elongation  of  any  tension  test  specimen  is 
less  than  that  specified  in  Section  8(a)  and  any  part  of  the  fracture 
is  outside  the  middle  third  of  the  gage  length,  as  indicated  by  scribe 
scratches  marked  on  the  specimen  before  testing,  a  retest  shall  be 
allowed. 

IV.    Permissible  Variations  in  Weight 

13.  Permissible  Variations.  —  The  weight  of  any  lot  of  bars  shall 
not  vary  more  than  5  per  cent  from  the  theoretical  weight  of  that  lot. 

One  class  of  high  carbon  steel  reinforcing  material  is  made 
by  heating  worn-out  steel  rails,  and  re-rolling  them  into  bars. 
This  is  generally  known  as  re-rolled  steel  or  rail  steel.  It  is 
usually  considerably  cheaper  than  that  made  from  new  steel, 
but  many  engineers  are  opposed  to  its  use  on  the  ground  that 
it  is  likely  to  contain  flaws,  and  therefore  is  unsafe.  The  author 
believes  that,  if  it  is  carefully  inspected  and  tested,  it  may  well 
be  used,  but  that  it  should  not  be  used  for  important  struc- 
tures without  test.  The  Specifications  given  by  the  American 
Society  for  Testing  Materials  for  the  physical  properties  and 
tests  of  re-rolled  steel  are  essentially  identical  with  those  for  the 
hard  grade  of  billet  steel,  as  heretofore  given,  except  that  no 
mention  is  made  of  cold-twisted  bars,  and  hot-twisted  bars  are 
specified  to  have  the  same  properties  as  other  deformed  bars. 

Forms  of  Reinforcing  Steel.  —  The  forms  in  which  steel  is 
most  used  for  reinforcing  are: 

1  Section  2  (b) .  —  If  desired,  cold-twisted  bars  may  be  purchased  on  the 
basis  of  tests  of  the  hot-rolled  bars  before  twisting,  in  which  case  such  tests 
shall  govern  and  shall  conform  to  the  requirements  specified  for  plain  bars  of 
structural-steel  grade. 


GENERAL  PRINCIPLES  99 

1.  Smooth  round  bars  or  wire, 

2.  Twisted  square  bars, 

3.  Round  or  square  bars  with  corrugations  or  projections  rolled  on 
their  surfaces, 

4.  Woven  or  welded  wire  fabric, 

5.  Expanded  sheet-metal  fabric. 

Examples  of  the  deformed  bars  and  of  the  steel  fabrics  are 
shown  in  Figs.  31,  32,  and  33. 


Fig.  31.  —  Typical  Deformed  Reinforcing  Bars. 

The  advantages  possessed  by  deformed  bars  (twisted  or  cor- 
rugated) over  smooth  bars  is  that  the  former  will  not  slip 
through  the  concrete  so  readily  as  the  latter,  when  they  are 


Fig.  32.  —  Woven  Wire  Reinforcing  Fabric. 

heavily  stressed.  This  is  an  important  matter  in  short  bars  of 
large  diameter,  but  is  much  less  important  in  small  bars  of 
considerable  length,  where  the  surface  of  the  bar  gripped  by 


100     CONCRETE  CONSTRUCTION  FOR  RURAL  COMMUNITIES 

the  concrete  is  much  greater  in  comparison  with  the  tension  in 
the  bar.  In  doubtful  cases,  the  resistance  of  smooth  bars  to 
slipping  may  be  considerably  increased  by  bending  back  three 
or  four  inches  of  the  ends  into  loops.  Twisted  square  bars  have 
been  very  extensively  used  for  reinforcing  concrete,  but  they  are 
now  being  displaced  by  other  types  of  deformed  bars.  Tests 
made  at  the  University  of  Illinois  show  that  the  twisted  square 
bar  begins  to  slip  in  the  concrete  at  an  even  lower  load  than  the 
plain  round  bar,  although  it  will  support  a  greater  load  before  it 
pulls  out  entirely.  Other  types  of  deformed  bars  gave  much 
more  favorable  results  in  the  tests. 


Fig.  33.  —  Expanded  Metal  Reinforcing  Fabric. 


Woven  wire  and  expanded  metal  fabrics  are  much  used  for 
reinforcing  floor  slabs,  tanks,  silos,  and  similar  structures.  When 
these  materials  are  used,  the  sectional  area  of  the  steel  extend- 
ing in  the  proper  direction  to  resist  the  stresses  should  be  made 
practically  equal  to  the  area  that  would  be  required  if  rods 
were  used.  Those  fabrics  which  have  the  principal  wires  or 
strands  straight,  without  loops,  kinks,  or  twists  of  any  kind, 
are  to  be  preferred. 

Expanded  metal  made  from  light  sheets  of  steel  is  now  much 
used  as  lath  for  cement  stucco  and  plaster  work.  A  special 
form  of  expanded  metal  lath  is  provided  with  deep  corruga- 
tions, or  ribs,  of  unexpanded  metal  at  intervals  in  the  width  of 


GENERAL  PBftfDl&LES 


101 


the  sheet.  These  ribs  make  #ie  ^eet -.sjtH?/ koij : rigid.', 'and  also 
give  it  considerable  reinforcing  value.'  When  this  material  is 
used  for  floors  and  roofs,  no  forms  are  necessary,  as  the  sheets 
are  stiff  enough,  with  a  few  braces,  to  support  the  weight  of 
the  concrete,  and  the  meshes  are  fine  enough  to  prevent  the 
concrete  from   running   through,   if  it  is  mixed   to  a  medium 


Fig.  34.  —  Expanded  Metal  Lath  with  Deep  Ribs  for  Stiffening  It. 

consistency.  After  the  concrete  on  the  upper  surface  has  set,  the 
lower  side  is  plastered  like  ordinary  lath.  When  the  ribs  do  not 
provide  enough  reinforcing  steel,  small  bars  may  be  used  in  addi- 
tion to  the  metal  lath.  This  material  is  also  used  for  walls,  both 
straight  and  curved,  the  concrete  in  such  cases  being  usually  ap- 
plied as  a  plaster.    Figure  34  illustrates  a  typical  lath  of  this  kind. 

Classes  of  Structures  in  which  Reinforced  Concrete  is  Used. 
—  The  principal  kinds  of  structural  elements  in  which  rein- 
forced concrete  is  used  are  hollow  cylinders,  beams  and  slabs, 
walls,  columns,  and  arches.  Illustrations  of  hollow  cylinders 
subjected  to  internal  pressure  are  water  tanks  and  silos.  Drain- 
age tile,  sewers,  and  some  culverts  illustrate  hollow  reinforced 
concrete  cylinders  subjected  to  external  pressure,  the  pressure 
of  the  earth.  Beams,  slabs,  walls,  and  columns  of  reinforced 
concrete  are  used  in  buildings  and  bridges.  Reinforced  concrete 
arches  are  often  used  in  bridges,  culverts,  sewers,  and  tunnels 
and  sometimes  in  buildings. 

In  all  the  cases  mentioned,  the  principal  purpose  of  the  steel 
is  to  resist  the  tensile  stresses.     In  some   cases,   especially  in 


102    CONCRETE  CONSTHUCTION  FOR  RURAL   COMMUNITIES 

columns. and  arches,,  it  is  useful  also  in  resisting  compression. 
To  enable  the  steel  to  fulfil  its  purpose  it  is  necessary  that  (1) 
a  sufficient  amount  of  it  be  provided;  (2)  that  it  be  in  a  form 
suitable  for  use;  and  (3)  that  it  be  properly  placed  in  the  con- 
crete. Those  not  familiar  with  the  fundamental  principles  of 
beam  action  may  use  much  more  steel  than  is  necessary,  but, 
not  placing  it  properly,  may  be  unsuccessful. 

Hollow  Cylinders  Subjected  to  Internal  Pressure.  —  When 
hollow  cylinders  are  subjected  to  an  internal  pressure  equally 
distributed  around  the  circumference,  as  is  the  case  in  water 
tanks  and  silos,  the  stress  is  almost  wholly  one  of  tension,  so 
that  concrete  without  reinforcing  would  be  entirely  unsuitable 
for  the  purpose.  The  walls  would  have  to  be  very  thick  to 
resist  the  outward  pressure,  and  the  cost  would  be  prohibitive. 
Hence  steel  reinforcing  is  depended  on  to  take  practically  all 
the  stress,  the  concrete  serving  only  to  hold  the  steel  in  place, 
to  protect  it  from  rusting,  and  to  retain  the  contents  of  the 
cylinder.  In  fact,  reinforced  concrete  cylinders  are  very  similar 
to  wooden  tanks,  the  steel  reinforcing  of  the  one  corresponding 
to  the  hoops  of  the  other,  and  the  concrete  of  the  one  to  the 
wooden  staves  of  the  other.  There  is  this  difference,  however. 
The  concrete  has  the  steel  embedded  in  it,,  so  that  the  latter  is 
protected  from  the  action  of  air  and  water  and  does  not  rust,  and 
the  concrete  itself  does  not  decay  like  the  wood.  The  reinforced 
concrete  tank  is  therefore  permanent  instead  of  temporary. 

Placing  the  Steel.  —  Since  the  stresses  in  concrete  cylinders 
are  not  bending  but  tension,  it  is  not  necessary  that  the  steel 
be  placed  close  to  one  face  or  the  other  as  in  most  other  cases 
of  reinforced  concrete.  Probably  the  best  place  for  the  steel  is 
near  the  middle  of  the  wall,  or  perhaps  a  little  nearer  the  outer 
edge,  in  order  to  give  it  the  best  protection  from  corrosion. 

It  is  not  necessary  that  the  ends  of  the  reinforcing  rods  be 
fastened  together  as  in  the  case  of  hoops  for  wooden  tanks,  but 
they  must  be  lapped  a  distance  of  about  sixty  times  the  diame- 
ter of  the  rods,  if  smooth  bars  are  used.  If  deformed  rods  are 
used,  or  if  the  ends  of  the  rods  are  bent  back  for  three  or  four 
inches  in  the  form  of  a  hook,  thirty  times  the  diameter  of  the 
rods  will  be  sufficient  lap.     Thus  |-inch  rods  should  be  lapped 


GENERAL  PRINCIPLES 


103 


60  X  !  =  38  inches  if  smooth  bars  are  used,  or  30  X  $  «■  19  inches 
if  deformed  or  hooked  bars  are  used. 

Beams  and  Slabs.  —  Beams  and  slabs  may  be  considered 
together,  as  a  slab  is  really  a  wide,  shallow  beam.  The  load 
which  must  be  supported  tends  to  bend  the  beam  or  slab.  This 
bending  tendency  causes,  and  is  resisted  by,  compressive  stresses 
on  one  side  of  the  beam  and  tensile  stresses  on  the  other  side. 
This  can  easily  be  demonstrated  in  a  timber  beam,  by  making 
a  saw  cut  near  the  middle.  If  the  cut  is  made  from  the  one 
side,  it  tends  to  close  up  and  pinch  the  saw,  thus  showing  that 
this  side  of  the  beam  was  in  compression.  If  the  cut  is  made 
from  the  opposite  side,  the  cut  tends  to  open  up,  allowing  the 


Load 


Reinforcing  Rods . 
on  Lower  Side 


Fig.  35.  —  Simple  Beam  Properly  Reinforced. 

saw  to  move  freely,  thus  showing  that  the  portion  cut  away  by 
the  saw  held  the  parts  together,  or  was  in  tension.  In  beams 
supported  at  both  ends,  with  the  load  pushing  or  pulling  down- 
ward, the  compression  will  be  at  the  upper  side  of  the  beam, 
while  the  tension  will  at  at  the  lower  side.  If  one  end  of  the 
beam  is  free,  the  other  end  being  fastened  so  that  it  cannot 
move,  with  the  load  pushing  or  pulling  down  on  the  free  end  of 


Reinforcing  Rods 
on  Upper  Side 


Fig.  36.  —  Cantilever  Properly  Reinforced. 

the  beam,  the  compression  will  be  at  the  lower  side  of  the  beam 
while  the  tension  is  at  the  upper  side.  This  kind  of  beam  is 
called  a  cantilever.  In  either  case  the  steel  reinforcing  must  be 
placed  near  the  surface,  on  the  tension  side  of  the  beam.  Fig- 
ures  35   and   36   show   the   proper   arrangement   in   these   two 


104     CONCRETE  CONSTRUCTION  FOR  RURAL   COMMUNITIES 

cases.     Figure  38  shows  the  effect  of  improper  location  of  the 
steel  on  the  strength  of  beams. 

Continuous  Beams.  —  Sometimes  beams  will  extend  over  more 
than  two  supports,  as  is  shown  in  Fig.  39.  The  dotted  lines 
show  in  an  exaggerated  manner  the  form  which  the  beam  takes 


Fig.  37.  —  Testing  a  Reinforced  Concrete  Beam. 

because  of  the  action  of  the  load.  It  can  easily  be  seen  that  ten- 
sion will  exist  on  the  lower  side  of  the  beam  near  the  middles  of 
the  spans  and  on  the  upper  side  of  the  beam  over  the  supports. 
Hence  the  reinforcing  steel  must  be  placed  close  to  the  lower 
side  of  the  beam  near  the  middles  of  the  spans  and  close  to  the 
upper  side  near  the  supports.  In  such  cases  it  is  common 
practice  to  bend  up  half  or  two-thirds  of  the  steel  rods  from 
the  bottom  of  the  beam  to  the  top,  at  an  angle  of  about  45 
degrees,  starting  from  the  bottom  at  points  about  one-third  of 
the  length  of  the  span  from  each  end.  Enough  additional  rods  are 
provided  over  the  supports  near  the  top  of  the  beam  to  make  the 
total  number  of  bars  here  equal  to  the  number  of  bars  near  the 
lower  surface  at  the  middle  of  the  span.  These  bars  should 
extend  one-third  of  the  span  length  on  each  side  of  the  supports. 


GENERAL  PRINCIPLES 


105 


It  can  be  seen  that  the  effect  will  be  the  same  as  in  the  case 
illustrated  in  Fig.  39,  if  heavy  girders,  instead  of  columns, 
form  the  supports  for  the  ends  of  the  beams.     In  any  case,  a 


Fig.  38.  —  Results  of  Tests  to  Illustrate  the  Effect  of  Improper  Location  of 
Steel  on  the  Strength  of  a  Beam. 

little  consideration  will  serve  to  show  on  which  side  the  concrete 
will  tend  to  pull  apart,  and  the  steel  should  be  placed  near  the 
surface  of  the  beam  on  that  side. 


rP*o    '• 

" :?^v^Nv ^  ^"~  ~~^t^???S^ 

#s 

Tgjgl 

~ 

mi 

Fig.  39.  —  Continuous  Beam  Properly  Reinforced. 

Depth  of  Embedment  of  Steel.  —  It  is  desirable  that  the 
steel  be  placed  as  close  to  the  surface  as  is  safe,  since  it  is  much 
more  effective  in  resisting  bending  when  near  the  surface  than 
when  near  the  middle  of  the  beam.     On  the  other  hand,  if  the 


106     CONCRETE  CONSTRUCTION  FOR  RURAL   COMMUNITIES 

rods  are  placed  too  near  the  surface,  there  will  be  danger  of 
cracking  or  splitting  off  the  concrete  below  the  rods  when  the 
beam  is  loaded.  There  will  also  be  danger  of  the  rods  rusting 
and  becoming  so  weakened  that  the  beam  will  lose  its  strength. 
In  buildings  and  other  places  where  the  concrete  may  be  endan- 
gered by  fire,  the  steel  must  be  placed  deep  enough  so  that  it 
will  not  be  materially  weakened  by  the  heat. 

A  very  thin  film  of  cement,  provided  it  is  unbroken,  will  pre- 
vent corrosion  of  the  steel.  On  account  of  the  danger  of  the 
formation  of  open  pockets  in  a  few  places,  however,  it  is  desir- 
able never  to  use  less  than  half  an  inch  of  concrete  outside  the 
rods,  to  prevent  corrosion,  especially  if  water  is  likely  to  come 
in  contact  with  the  surface. 

To  prevent  the  concrete  from  splitting  off,  and  to  permit  it  to 
be  well  worked  into  place  between  and  below  the  rods,  the  latter 
should  be  spaced  so  there  is  a  distance  of  not  less  than  one  and 
one-half  times  the  diameter  of  the  rods,  in  the  clear,  between 
adjacent  rods,  with  a  distance  of  not  less  than  one  diameter 
in  the  clear  between  the  rods  and  the  bottom  of  the  form. 
Thus  if  f-inch  rods  are  used,  they  should  be  spaced  not  less 
than  1|  X  f  =  1|  inches  apart  in  the  clear,  or  1J  +  f  =  If  inches 
from  center  to  center,  while  the  clear  distance  between  the  bars 
and  the  bottom  of  the  beam  should  be  not  less  than  f  inch. 

If  the  rule  given  in  the  above  paragraph  is  followed,  with  a 
minimum  distance  of  one-half  inch  between  the  rods  and  the 
forms,  this  will  be  satisfactory  for  most  cases  coming  within 
the  scope  of  this  book.  In  important  structures  subject  to  fire 
risk,  the  embedment  given  above  would  be  insufficient  for  pro- 
tection from  fire,  a  minimum  depth  of  one  and  one-half  to  two 
inches  being  required. 

The  steel  should  be  well  secured  in  its  proper  position,  so 
that  it  may  not  be  displaced  by  the  concrete,  or  by  the  work- 
man while  the  concrete  is  being  placed.  The  rods  in  beams 
should  be  well  wired  to  each  other  and  to  the  forms.  Special 
care  should  be  taken  that  the  slab  rods  do  not  sag  too  low. 
Wooden  laths  may  be  used  to  hold  up  the  rods,  if  they  are  re- 
moved as  the  concrete  reaches  them.  A  better  way  is  to  use  a 
few  steel  rods  of  the  proper  size,  placed  at  right  angles  to  the 


GENERAL  PRINCIPLES 


107 


main  reinforcing,  or  some  form  of  patented  support,  which  may- 
be left  in  place  when  the  concrete  is  poured.  Figure  40  shows 
such  a  support.  If  the  cross  rods  are  well  wired  to  the  main 
rods,  supports  will  be  needed  at  only  a  few  points,  and  these 
may  be  supplied  by  pieces  of  flat  stone  or  gravel. 

Columns.  —  The  principal  stress  in  columns  is  of  course  com- 
pressive, and,  as  concrete  is  strong 
in  compression,  it  might  seem  that 
steel  reinforcement  would  not  be 
needed.  In  fact,  short  concrete  col- 
umns or  piers  are  not  infrequently 
built  without  reinforcing.  If,  how- 
ever, there  should  be  any  side  thrust 
on  the  column,  or  if  the  load  should 
not  act  exactly  through  the  center 
of  the  column,  a  bending  would  be 
produced  which  would  tend  to  make 
the  column  buckle  sidewise.  This 
would  give  rise  to  tensile  stresses  in 
the  concrete,  on  the  side  buckling 
outward.  To  resist  these  stresses, 
some  steel  is  ordinarily  used  in  the  columns.  As  the  column 
may  buckle  in  any  direction  it  is  necessary  that  it  be  rein- 
forced on  all  sides.  A  rod  is  often  placed  in  each  corner.  The 
steel  also  aids  in  carrying  the  compressive  stresses,  and  the  col- 
umn may  be  made  smaller  than  if  no  steel  were  used.  This 
is  often  an  important  matter  in  buildings  where  floor  space  is 
very  valuable. 

The  action  of  the  steel  in  concrete  columns  is  similar  to  that 
in  beams,  and  therefore  the  same  remarks  will  apply  as  to  the 
distance  it  should  be  placed  from  the  surface  of  the  concrete.  If 
the  clear  distance  of  the  rods  from  the  surface  is  made  one  and 
one-half  times  the  diameter  of  the  rod,  this  should  be  sufficient, 
except  where  the  necessity  for  fire  protection  requires  a  greater 
distance. 

Sometimes  the  steel  columns  are  reinforced  by  means  of 
wire  hoops  or  spirals.  These  tend  to  prevent  the  bulging  out 
which  necessarily  takes  place  when  the  columns  shorten  under 


Fig.  40.  —  Support  for  Holding 
Reinforcing  Steel  in  Position. 


108    CONCRETE  CONSTRUCTION  FOR  RURAL  COMMUNITIES 


the  action  of  loads.  The  hoops  are  not  effective  in  resisting  a 
tendency  to  bend;  hence  it  is  always  desirable  that  some  rods 
be  used  with  the  hoops.  These  rods  are  usually  placed  just  in- 
side the  hooping  and  often  serve  as  spacers  to  hold  the  latter 
the  right  distance  apart. 

Arches  and  Hollow  Cylinders  Subjected  to  External  Pressure. 
—  In  arches,  the  stress  is  a  combination  of  bending  and  com- 
pression. Steel  is  often  used  near  both  the 
outer  and  the  inner  surfaces,  to  carry  the 
tension  due  to  bending,  as  this  may  be  first 
on  the  one  side  and  then  on  the  other. 

In  sewers  and  drainage  tile,  the  load  is 
due  chiefly  to  the  weight  of  the  earth  above 
the  tile.  This  tends  to  cause  the  top  and 
the  bottom  to  flatten,  and  the  sides  to  bulge 
out.  This  produces  a  tension  or  tendency 
to  crack  on  the  inside  of  the  wall  at  the 
top  and  bottom,  and  on  the  outside  at  the 
sides,  as  shown  in  Fig.  42.  This  tension  is 
provided  for  by  placing  the  steel  in  either 
of  the  manners  shown  in  Fig.  43. 

Differences  in  Materials  and  Methods 
Used  for  Reinforced  Concrete.  —  The 
stresses  in  reinforced  concrete  are  usually 
much  higher  than  in  plain  concrete.  It  is 
therefore  important  that  only  the  best  ma- 
terials and  rich  mixtures  be  used,  and  that 
every  care  be  taken  to  get  a  strong  dense 
concrete,  free  from  all  open  pockets  and 
other  imperfections. 

The  coarse  aggregate  should  be  a  firm 
hard  stone,  or  a  clean  gravel,  screened  over  a  J-inch  screen.  The 
size  of  the  stone  or  gravel  should  be  somewhat  smaller  than  is 
usually  necessary  for  plain  concrete,  on  account  of  the  difficulty 
in  getting  it  worked  into  place  around  the  reinforcement  so  that 
no  open  pockets  will  be  left.  The  thickness  of  the  members  is 
frequently  much  less  than  is  usual  in  unreinforced  work,  and  this 
is  an  additional  reason  for  using  a  smaller  coarse  aggregate.      In 


Fig.  41.  — Spiral   Col- 
umn Reinforcing. 


GENERAL  PRINCIPLES 


109 


average  work  a  stone  or  gravel  which  would  pass  a  1-inch 
mesh  screen  is  satisfactory,  and  even  a  little  coarser  aggregate 
will  sometimes  do,  if  care  is  taken  in  working  it  into  place. 
Still  finer  aggregate  may,  in  some  cases,  be  desirable. 

The  concrete  for  reinforced  work,  in  order  that  it  may  be 
easily  worked  into  place  around  the  steel  and  into  the  corners, 
should  usually  be  mixed  to  the 


0&/W 


IB 


i 


consistency  designated  in  Chap- 
ter VI  as  a  wet  mixture.  Com- 
pacting should  be  done  with  a 
spade,  which  should  be  worked 
up  and  down  in  the  concrete 
sufficiently  to  free  the  latter 
from  all  air,  and  to  insure  that 
the  concrete  has  entirely  filled 
the  space  below  and  around  the 
steel.  Care  should  be  taken,  how- 
ever, not  to  displace  the  latter. 
Columns  should,  if  possible, 
be  poured  complete  at  one  time 
and  then  a  few  hours  or  more  should  be  given  for  these  to  set- 
tle before  the  floor  above  is  poured.     When  work  on  floors  is 


Fig.  42.  —  Method  of  Collapse  of 
Sewer  under  Earth  Pressure. 


Fig.  43.  —  Methods  of  Reinforcing  Concrete  Tile. 

stopped,  the  beams  and  slabs  should  be  stopped  near  the  middle 
—  not  at  one  end  as  might  be  supposed.  If  the  work  is  stopped 
near  the  ends  of  the  beams,  diagonal  cracks  which  may  be  dan- 
gerous are  likely  to  form  at  these  places.  The  full  depth  of  all 
beams  and  slabs  must  be  poured  at  one  time  as  far  as  the  work 
is  carried. 


110     CONCRETE  CONSTRUCTION  FOR  RURAL   COMMUNITIES 

In  much  reinforced  concrete  work,  as  in  floors,  the  weight  of 
the  concrete  itself  causes  a  large  part  of  the  total  load  which 
the  member  is  required  to  carry.  In  such  cases  it  is  highly 
important  that  the  forms  be  left  in  place  until  the  concrete  has 
had  ample  time  to  harden.  At  least  two  weeks  should  be  allowed, 
and  more  if  a  load  is  applied  on  top  of  the  floor. 

Full  loads  should  not  be  applied  to  reinforced  structures 
until  at  least  one  month  after  the  concrete  is  poured.  Most 
structures  are  designed  on  the  basis  of  the  strength  the  concrete 
will  possess  at  the  end  of  one  month.  Hence,  if  they  are  loaded 
earlier,  the  concrete  may  not  yet  be  strong  enough  to  with- 
stand the  stresses  developed.  If  it  is  necessary  to  apply  the 
load  earlier,  as  it  may  be  at  times,  braces  should  be  put  under 
the  middle  of  the  slabs,  beams,  and  girders.  If  the  forms  are 
left  in  place  in  such  cases  until  the  end  of  one  month,  they  will 
offer  material  assistance. 


CHAPTER  VIII 
STRENGTH    OF    REINFORCED    CONCRETE 

While  the  design  of  complex  reinforced  concrete  structures 
is  a  complicated  matter  which  should  be  left  to  competent 
concrete  engineers,  and  a  discussion  of  which  is  therefore  be- 
yond the  scope  of  this  book,  yet  enough  may  be  given  here  to 
enable  the  reader  to  make  intelligent  use  of  steel  for  reinforcing 
the  simpler  structures  most  often  met  with  in  rural  communities. 

Stresses  Used  in  Reinforced  Concrete.  —  It  is  usual  in  rein- 
forced concrete  design  to  base  the  stresses  in  the  concrete  on 
the  strength  that  the  latter  will  possess  at  the  end  of  four  weeks, 
as  it  will  usually  be  necessary  to  use  the  structure  at  about 
this  time.  If  the  maximum  load  must  be  applied  at  an  earlier 
date  than  this,  then  the  strength  of  the  concrete  at  the  end  of 
the  shorter  period  should  form  the  basis  of  design.  The  increase 
in  strength  in  longer  periods  will  then  give  added  security  to 
the  structure. 

In  Chapter  VI  the  crushing  strength  of  1:2:4  concrete  at  the 
end  of  28  days  was  given  as  about  2000  to  2200  lb.  per  sq.  in. 
when  gravel  or  the  harder  stones  are  used.  It  would  not  do, 
however,  to  use  such  high  values  in  the  design,  for,  if  the  con- 
crete were  not  quite  so  strong  as  expected,  or  if  the  load  were 
a  little  greater,  or  if  the  structure  received  a  jar,  then  failure 
would  occur.  Also,  repetitions  of  the  load  would  cause  failure 
at  stresses  much  lower  than  the  value  given,  just  as  a  piece 
of  wire  or  a  bar  of  steel  can  be  broken  by  repeated  bending 
back  and  forth.  For  these  reasons,  the  values  of  strength  found 
by  tests  must  be  divided  by  some  number,  called  a  factor  of 
safety,  to  get  values  for  the  safe  stresses  to  be  used  in  design. 
This  factor  differs  for  different  materials  and  for  different  types 
of  structural  elements. 

As  it  is  impossible  to  tell  in  advance  just  how  strong  a  given 

111 


112    CONCRETE  CONSTRUCTION  FOR  RURAL  COMMUNITIES 

concrete  will  be,  it  is  customary  to  assume  a  strength  of  2000 
lb.  per  sq.  in.  for  good  1:2:4  concrete  at  the  end  of  28  days, 
and  to  apply  the  factors  of  safety  to  this  value.  If  in  any  case 
there  is  reason  to  believe  that  a  lower  strength  will  be  obtained, 
then  the  working  stresses  should  be  correspondingly  reduced. 
The  factor  of  safety  for  the  concrete  of  columns  and  piers  is 
usually  taken   at   about  4.5.     The  working  stress  is  therefore 

— —  or,  say,  450  lb.  per  sq.  in.  For  beams  and  slabs  a  fac- 
tor of  about  3  is  used,  giving   — — ,  or  about   650  lb.  per  sq. 

o 

in.,  as  the  safe  working  stress. 

The  factor  of  safety  usually  used  for  steel  in  tension  is  from 
4  to  5,  giving  maximum  values  for  working  stresses  of  12,000 
to  16,000  lb.  per  sq.  in.  for  medium  steel,  and  from  16,000  to 
18,500  lb.  per  sq.  in.  for  high  carbon  steel.  It  is  not  generally 
thought  desirable  to  use  higher  stresses  than  16,000  lb.  per  sq. 
in.,  and  14,000  lb.  per  sq.  in.  is  probably  better  for  medium 
steel.  Table  V  will  be  found  convenient  in  selecting  the  sizes 
of  rods  to  use. 

Hollow  Cylinders  Subjected  to  Internal  Pressure.  —  In  hol- 
low cylinders  subject  to  internal  pressure,  the  steel  is  depended 
on  to  take  care  of  all  the  principal  stresses  by  hoop  tension. 
It  can  easily  be  seen  that  more  steel  will  be  required  near  the 
bottom  of  a  cylinder  than  near  the  top,  on  account  of  the 
greater  pressure  existing  here.  The  following  rule  may  be  used 
to  find  the  stress  in  the  steel,  at  any  distance  from  the  top  of  a 
cylinder,  due  to  pressure  from  a  liquid:  Multiply  the  height, 
in  feet,  of  the  liquid  above  the  section  under  consideration,  by 
the  weight  of  a  cubic  foot  of  the  liquid;  multiply  this  by  the 
diameter  of  the  cylinder  in  feet;  and  divide  the  product  by 
twice  the  area  of  the  cross-section  of  steel  in  square  inches  per 
foot  of  height  of  the  wall.  The  spacing  of  the  reinforcement 
should  not  be  made  greater  than  two  and  a  half  to  three  times 
the  thickness  of  the  wall.  The  rule  may  be  expressed  alge- 
braically as  follows: 


STRENGTH  OF  REINFORCED  CONCRETE 


113 


02 

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114     CONCRETE  CONSTRUCTION  FOR  RURAL   COMMUNITIES 

hwd 
where  2  a 

s  =  stress  in  the  steel  in  pounds  per  square  inch, 

h  =  height   of   the   liquid,    in   feet,    above   the   section 

under  consideration, 
w  =  weight  of  the  liquid  in  pounds  per  cubic  foot, 

=  62.5  for  water, 
d  =  diameter  of  the  cylinder  in  feet, 
a  =  area  of  the  cross-section  of  the  steel,  in  square  inches 
per  foot  of  height,  at  the  section  under  consideration. 

Problem  1.  —  Find  the  stress  in  the  reinforcing  of  a  water  tank  8  feet  in  di- 
ameter, at  a  point  10  feet  from  the  top,  if  the  reinforcing  at  this  depth  consists 
of  f-inch  round  rods  spaced  7  inches  from  center  to  center. 

Solution.  —  The  area  of  a  f-inch  round  rod  is  0.11  sq.  in.,  so  there  are  0.11 

Oil  x  12 
sq.  in.  of  steel  for  each  7  inches  in  height,  or  — '■ — = =  0.19  sq.  in.  per  foot  of 

height.     (See  also  Table  VIII.)    Then  the  stress  in  the  steel  is  equal  to 

10  x  62.5  x  8 


2  x  0.19 
which  is  safe. 


=  13.160  lb.  per  sq.  in., 


If  we  wish  to  find  how  much  steel  is  required  for  reinforcing 
a  tank,  we  may  proceed  in  a  similar  manner.  Multiply  the 
height,  in  feet,  of  the  liquid  above  the  section  being  considered, 
by  the  weight  of  a  cubic  foot  of  the  liquid;  multiply  this  by  the 
diameter  of  the  tank  in  feet;  and  divide  the  product  by  twice 
the  allowable  stress  in  the  steel  in  pounds  per  square  inch.  The 
result  will  be  the  area  in  square  inches  of  steel  required  per  foot 
of  height  of  the  wall,  or,  algebraically, 

hwd 

in  which  formula  the  letters  have  the  same  meaning  as  before. 

Problem  2.  —  Find  the  amount  of  steel  required  to  reinforce  a  cylindrical 
water  tank  12  feet  in  diameter  at  a  point  6  feet  below  the  top,  a  stress  in  the 
steel  of  14,000  lb.  per  sq.  in.  being  allowable. 

Solution.  —  Applying  the  rule,  we  have  the  area  of  steel  per  foot  of  height 
equal  to 

6  X  62.5  X  12      n  _ 
^X  14,000    =°-16^in' 


STRENGTH  OF  REINFORCED   CONCRETE  115 

/ 
By  Table  VIII  this  requires  a  spacing  of  8£  inches  from  center  to  center,  if 
f-inch  round  rods  are  used.    Larger  or  smaller  rods  will  do,  if  they  are  spaced 
to  give  an  equal  amount  of  steel,  as  shown  by  the  table,  provided  the  spacing 
is  made  not  greater  than  2\  to  3  times  the  thickness  of  the  wall. 
At  a  depth  of  10  feet  below  the  top,  the  amount  of  steel  required  is 

10  x  62.5  x  12      _  npo 
2x14,000      -  0.268  sq.m. 

which  requires  a  spacing  of  4|  inches  from  center  to  center,  if  f-inch  rods  are 
used. 

As  the  concrete  walls  serve  merely  to  retain  the  water  and 
protect  the  steel,  they  may  be  made  of  any  convenient  thickness. 
Six  or  eight  inches  is  usual  except  in  very  large  or  tall  cylinders. 

In  deep  tanks  it  is  desirable  to  use  some  vertical  rods  to  pre- 
vent possible  horizontal  cracks.  Round  rods,  J  inch  or  |  inch 
in  diameter,  spaced  18  to  24  inches  on  centers,  will  usually  be 
sufficient. 

In  silos  and  other  hollow  cylinders  containing  solid  or  partly 
solid  matter,  the  pressure  against  the  walls  is  less  than  that  pro- 
duced by  liquids,  and  the  above  method  of  calculation  will  give 
an  excess  of  steel. 

Columns.  —  Concrete  columns  should  not  be  made  longer 
than  15  times  their  diameter  (or  width  of  one  side,  if  square) 
between  points  of  support.  If  made  only  six  to  eight  times 
their  diameter,  they  need  not  be  reinforced.  If  they  are  longer, 
an  amount  of  reinforcement  equal  to  1  per  cent  or  more  of  the 
area  of  the  column  should  be  used.  This  should  be  distributed 
into  one  rod  near  each  corner,  and  the  rods  should  be  tied 
together  by  wires  at  intervals  of  about  a  foot. 

The  central  load  which  a  column  can  safely  carry  may  be 
calculated  by  multiplying  the  area  of  the  section  by  the  safe 
unit  stress  (450  lb.  per  sq.  in.  for  good  1:2:4  concrete).  This 
method  neglects  the  increase  of  strength  due  to  the  reinforcing 
steel,  which  is  sometimes  taken  into  account  by  engineers, 
but  it  is  believed  to  be  best  for  those  not  thoroughly  familiar 
with  concrete  design.  As  a  small  eccentricity  in  the  load  will 
produce  a  large  increase  in  stress,  great  care  must  be  taken  to 
see  that  the  load  is  applied  exactly  through  the  center  of  the 
column. 


116     CONCRETE  CONSTRUCTION  FOR  RURAL   COMMUNITIES 

Problem  1.  —  Find  the  safe  load  on  a  1 :  2:  4  concrete  column  10"  x  10"  x  12' 
long.    How  much  steel  should  be  used  for  reinforcing  the  column? 

Solution.  —  The  safe  load  will  be  10  x  10  x  450  -  45,000  lb.    The  length 

12  x  12 
is  —      -   =  14.4  times  the  width  of  one  side,  hence  reinforcing  rods  must 

be  used.    Their  combined  area  may  be  .01  x  10  X  10  =  1.00  sq.  in.    Use  one 
|-inch  square  rod  or  one  f-inch  round  rod  near  each  corner. 

Problem  2.  —  Design  a  column  to  carry  a  safe  load  of  40  tons,  if  its  unsup- 
ported length  is  16  feet. 

Solution.  —  The  area  of  the  column  section  must  be r^ =  178  sq.  in., 

450  M        ' 

or  the  column  must  be  13.3  or,  say,  13f  inches  square.    This  makes  the  length 

equal  to  — ^ol —  =  1^.2  times  one  side,  which  is  allowable.    Use  about  1  per 

cent  of  steel,  or  .01  x  13|  x  13|  =  1.82  sq.  in.    This  requires  one  ^-inch 
square,  or  one  f-inch  round,  rod  near  each  corner. 

If  the  length  of  the  column  had  been  20  feet,  the  size  would  have  had  to  be 

20  x  12 
increased  to  — r= —  =  16  inches  square,  in  order  to  keep  the  length  from  being 
10 

more  than  15  times  the  width  of  one  side. 

In  buildings  where  there  is  considerable  fire  risk,  the  outer 
one  and  one-half  inches  all  around  the  column  should  be  re- 
garded as  fire-proofing  only,  and  should  not  be  taken  into 
account  in  computing  the  safe  load.  In  such  cases  the  reinforc- 
ing bars  should  be  placed  two  or  two  and  one-half  inches  in  the 
clear  from  the  outside  of  the  column,  so  that  they  will  be  pro- 
tected from  the  fire.  For  illustration,  in  problem  2,  if  the  column 
is  to  be  used  in  a  fire-proof  building  it  must  be  made  13 J  + 
2  X  1J  =  16|  inches  square,  to  carry  safely  40  tons  in  a  hot  fire. 

Beams  and  Slabs.  —  In  determining  the  strength  of  beams 
and  slabs,  the  concrete  below  the  center  of  the  steel  is  neglected 
and  the  effective  depth  is  taken  as  equal  to  the  distance  from 
the  upper  surface  to  the  center  of  the  steel.  The  amount  of 
steel  generally  used  is  1  per  cent  of  the  area  of  the  beam  or 
slab  above  the  steel,  or  a  little  less.  Thus  if  a  beam  is  8  inches 
wide  and  15  inches  deep,  with  the  center  of  the  steel  2  inches 
above  the  bottom  of  the  beam,  the  effective  depth  of  the  beam 
is  only  13  inches,  and  the  area  of  steel  used  should  be  about 
.01X8X13  =  1.04  sq.  in.  This  would  correspond  to  two  fl- 
inch round  rods,  which  have  a  combined  area  of  1.037  sq.  in. 

Similarly,  if  the  total  depth  of  a  slab  is  5  inches  and  the 


STRENGTH  OF  REINFORCED  CONCRETE  117 

center  of  the  steel  is  f  inch  above  the  bottom,  the  effective  depth 
is  only  4f  inches;  and  0.01  X  4|  X  12  =  0.51  sq.  in.  of  steel  may 
be  used  per  foot  of  width,  corresponding  to  |-inch  round  rods  4J 
inches  from  center  to  center  or  f-inch  round  rods  7J  inches  on 
centers. 

Rectangular  Beams.  —  Where  not  more  than  1  per  cent  of 
steel  is  used,  and  where  the  length  is  not  less  than  twelve 
times  the  effective  depth,  the  safe  uniformly  distributed  load 
which  can  be  carried  by  a  simple  rectangular  beam  supported 
at  both  ends,  of  1:2:4  concrete  not  less  than  one  month  old, 
can  be  found  by  the  following  rule.  It  is  based  on  a  stress  of 
14,000  lb.  per  sq.  in.  in  the  steel  and  about  650  lb.  per  sq. 
in.  in  the  concrete. 

Multiply  the  depth  of  the  steel  from  the  surface  of  the  beam 

by  8100  times  the  area  of  the  steel,  and  divide  the  product  by 

the  length  of  the  beam  in  feet.    The  result  will  be  the  safe  load 

in  pounds,  including  the  weight  of  the  beam.     To  find  the  load 

which  can  safely  be  placed  on  top  of  the  beam,  subtract  the 

weight  of  the  beam  itself.     The  weight  of  the  concrete  may  be 

taken  as   150  lb.  per  cubic   foot.     The  rule  may  be  expressed 

algebraically  as 

8100Asd      __, 
w  =  — W 

where         w  =  the  safe  uniformly  distributed  load,  in  pounds, 
which  can  be  placed  on  top  of  the  beam, 
As  =  area  of  the  cross-section  of  the  steel,  in  square 

inches, 
d  =  depth  from  the  top  of  the  beam  to  the  center 

of  the  steel,  in  inches, 
L  =  length  of  beam  in  feet, 
and  W  =  weight  of  the  beam  itself,  in  pounds. 

It  should  be  especially  noted  that  this  rule  does  not  apply  to  a 
beam  containing  more  than  1  per  cent  of  steel  nor  to  a  beam  the 
length  of  which  is  less  than  twelve  times  its  effective  depth. 

Problem  1.  —  Find  the  safe  uniformly  distributed  load  which  can  be  placed 
on  a  concrete  beam  9  inches  wide,  16  inches  deep,  and  15  feet  long,  if  it  is 
reinforced  by  means  of  two  I-inch  round  rods,  the  center  of  which  is  2  inches 
above  the  bottom  of  the  beam. 


118     CONCRETE  CONSTRUCTION  FOR  RURAL  COMMUNITIES 

Solution.  —  The  effective  area  of  the  beam  is  equal  to  9  x  14  =  126  sq.  in. 

1  203  x  100 
The  area  of  steel  is  1.203  sq.  in.,  so  that  the  percentage  of  steel  is  - — ^ 

15  X  12 
=  0.95.      The  length  is  — ^1 —  -  12.8  times  the  effective  depth.     As  the 

reinforcement  is  less  than  1  per  cent,  and  the  beam  is  not  too  short,  the  above 

rule  will  apply.    Therefore  the  total  load  which  can  be  carried,  including  the 

8100  x  1  203  x  14 
weight  of  the  beam,  will  be  equal  to  -        — j^ =  9100  pounds.     The 

weight  of  the  beam  itself  will  be  150  x  -^-  X  15  =  2250  pounds.  Conse- 
quently the  safe  load  which  can  be  placed  on  the  beam  is  9100  -  2250  -  6850 
pounds. 

If  the  size  of  a  beam  to  carry  a  given  load  is  to  be  found,  it 
may  be  assumed,  and  the  strength  calculated  by  the  above  rule. 
If  this  beam  is  found  to  be  too  weak  or  too  strong,  a  larger  or 
a  smaller  size  should  then  be  assumed,  and  the  strength  again 
calculated,  this  process  being  continued  until  a  size  is  found 
which  is  safe,  and  yet  not.  excessive.  It  will  usually  be  desirable 
to  make  the  breadth  of  rectangular  beams  about  §  to  J  of  their 
effective  depth. 

Problem  2.  —  Design  a  beam  to  carry  safely  a  uniformly  distributed  load 
of  10,000  pounds  on  a  span  of  18  feet. 

Solution.  —  Since  this  beam  must  carry  a  larger  load,  with  a  longer  span, 
than  the  beam  of  the  example  above,  it  must  be  heavier.  Assume  a  width 
of  12  inches  and  an  effective  depth  of  18  inches.  This  depth  is  just  TV  of  the 
span.  If  1  per  cent  of  steel  were  used,  this  would  be  .01  x  12  x  18  =  2.16  sq. 
in.  Try  two  one-inch  square  rods  with  an  area  of  2.0  sq.  in.,  a  .little  less  than 
1  per  cent. 

The  total  safe  load,  including  the  weight  of  the  beam,  is  by  the  rule  given 
above, 

8100x2x18      tannA  , 
— =  16,200  pounds. 

lo 

The  weight  of  the  beam  itself,  if  two  inches  of  concrete  are  allowed  below 
the  steel,  making  the  total  depth  of  the  beam  20  inches,  is 

150  x  12^2°  x  18  =  4500  pounds. 

Hence  the  safe  load  which  can  be  placed  on  top  of  the  beam  is 
16,200  -  4,500  -  11,700  pounds. 
As  this  is  more  than  is  required,  a  somewhat  smaller  size,  say  11  inches  by 
18  inches,  may  be  assumed,  and  its  strength  calculated.    Proceeding  as  before, 
we  find  that,  with  three  f-inch  round  rods,  the  safe  load  on  top  of  this  beam  is 
10,500  pounds.    This  is  close  enough,  and  the  beam  may  be  used. 


STRENGTH  OF  REINFORCED  CONCRETE  119 

If  beams  are  required  to  be  a  little  deeper  than  one-twelfth  of 
their  length,  the  method  given  above  may  be  used  in  designing 
them,  provided  several  stirrups  of  f-inch  or  f-inch  rods  are 
placed  near  each  end  and  spaced  a  distance  of  about  half  the 
depth  of  the  beam  from  each  other,  with  the  first  one  an  equal 
distance  from  the  end  of  the  beam.  One-third  to  one-half  of 
the  main  reinforcing  rods  should  also  be  bent  up  at  an  angle  of 
about  45  degrees,  beginning  at  points  from  one-fourth  to  one- 
third  the  length  of  the  span  from  the  ends.  Deformed  bars 
should  be  used  in  all  such  deep  heavy  beams. 

Sometimes  the  beams  must  be  designed  to  carry  a  load  concen- 
trated at  the  middle  of  the  span,  instead  of  uniformly  distributed. 
In  such  cases  one  may  proceed  to  find  the  safe  uniformly  dis- 
tributed load  which  can  be  put  on  the  top  of  the  beam,  and  then 
divide  this  by  2  to  find  the  safe  load  concentrated  at  the  middle. 

Floor  and  Roof  Slabs.  —  Floor  and  roof  slabs  may  be  consid- 
ered equivalent  to  a  large  number  of  beams  laid  side  by  side, 
and  hence  the  rule  given  above  may  be  used  to  find  their  safe 
loads.  It  is  convenient  to  make  the  calculations  on  the  basis 
of  a  width  of  strip  of  12  inches,  though  any  other  width  would 
give  the  same  results.  Slabs  will  rarely  be  shorter  than  twelve 
times  their  effective  depth,  so  stirrups  will  seldom,  if  ever,  be 
required.  The  spacing  of  the  main  reinforcing  rods  should 
never  be  greater  than  2\  to  3  times  the  effective  depth  of  the 
slab. 

It  is  desirable  to  have  some  light  rods  at  right  angles  to  the 
main  reinforcing,  to  assist  in  distributing  concentrated  loads, 
and  to  resist  stresses  caused  by  shrinkage  and  changes  of  tem- 
perature. Round  rods  \  inch  to  f  inch  in  diameter,  spaced 
about  24  inches  on  centers,  are  commonly  used.  This  steel  is  not 
taken  into  account  when  the  percentage  of  reinforcement  is  figured. 

Problem  1.  —  Find  the  safe  uniformly  distributed  load  on  a  flat  slab  floor 
of  4£  inches  total  thickness  and  10  feet  span,  if  it  is  reinforced  with  1  per  cent 
of  steel,  with  a  distance  of  \  inch  from  the  center  of  the  steel  to  the  bottom  of 
the  slab. 

Solution.  —  Consider  a  section  of  the  slab  one  foot  wide,  and  treat  it  as  a 
beam.  The  effective  depth  of  the  slab  is  4£  -  f  =  3|  inches,  and  the  effective 
area  of  the  concrete  in  this  width  is,  therefore,  3|  x  12  =  45  sq.  in.    Hence 


120     CONCRETE  CONSTRUCTION  FOR  RURAL  COMMUNITIES 

the  area  of  steel  required  per  foot  of  width  of  the  slab  is  1  per  cent  of  this,  or 
0.45  sq.  in.  Table  VIII  shows  that  this  corresponds  to  £-mch  round  rods  5£ 
inches  from  center  to  center,  or  f-inch  round  rods  8  inches  on  center.  Then 
applying  the  rule  given  above,  the  total  load  the  slab  can  carry  for  each  foot  of 
width,  including  the  weight  of  the  slab,  is  810°  x  0.45  *  3f  =  137Q  pmmdg 

The  weight  of  this  width  of  slab  is  150  x    **       x  10  =  560  pounds,  so  the 

load  which  can  be  safely  put  on  top  of  the  floor  is  1370  -  560  =  810  pounds, 
for  each  foot  of  width,  or  Vo0  =81  pounds  per  square  foot. 

Problem  2.  —  Find  the  depth  of  slab  and  amount  of  reinforcing  required 
to  carry  a  load  of  150  pounds  per  sq.  ft.,  in  addition  to  the  weight  of  the  slab, 
on  a  span  of  12  feet. 

Solution.  —  Assume  a  total  depth  of  slab  of  7  inches,  with  the  center  of  the 
steel  1  inch  above  the  bottom.  Then  the  effective  depth  is  7  -  1  =  6  inches. 
One  per  cent  of  steel  will  require  for  each  foot  of  width  of  slab  .01  x  6  x  12 
-  0.72  sq.  in.,  which  can  be  supplied  by  f-inch  round  rods  spaced  1\  inches 
on  centers.  Then  the  safe  load,  including  the  weight  of  the  slab,  which  each 
foot  of  width  can  carry  is, 

8100  x  0.72  x  6      ooon  , 
=  2 =  2920  pounds. 

The  slab  will  weigh,  for  this  width, 

150  x  -y^P  X  12  =  1050  pounds, 

making  the  safe  load  on  top  of  the  slab  2920  -  1050  =  1870  pounds  per  foot 

1870 
of  width  or  — —  =  156  pounds  per  square  foot.    This  is  close  enough  to  the 

given  load  for  all  purposes.  Had  the  safe  load  come  out  too  low  or  too  high, 
another  calculation  on  a  slightly  stronger  or  weaker  assumed  size  would  have 
been  required. 

Continuous  Beams.  —  Beams  and  slabs  which  are  continuous 
over  several  supports  should  have  about  half  or  two-thirds  of 
the  steel  bent  up  at  an  angle  of  45  degrees  at  about  half  or 
two-thirds  of  the  distance  from  the  center  to  the  end  of  the 
span,  as  discussed  in  Chapter  VII,  and  should  have  additional 
rods,  of  length  equal  to  two-thirds  the  span,  provided  near  the 
upper  surface,  above  the  supports,  to  make  the  total  amount 
of  steel  here  equal  to  that  in  the  middles  of  the  spans.  If  this 
is  not  done,  unsightly  and  possible  dangerous  cracks  are  likely 
to  form  over  the  supports.  In  slabs  reinforced  with  wire  fabric 
or  expanded  metal,  all  this  is  usually  bent  up  over  the  supports. 


STRENGTH  OF  REINFORCED   CONCRETE 


121 


The  strength  of  continuous  beams  and  slabs,  reinforced  as 
directed,  is  25  to  50  per  cent  greater  than  where  the  reinforcing 
over  the  supports  is  not  provided. 


Fig.  44.  —  Interior  of  Reinforced  Concrete  Building,  Showing  Arrangement 
of  Slabs,  Beams,  Girders  and  Columns. 

T-Beams.  —  Concrete  beams  are  very  frequently  cast  in  one 
piece  with  the  slabs.  Such  beams,  called  T-beams,  may  be 
given  as  much  larger  amount  of  steel  than  ordinary  rectangular 
beams  of  the  same  size,  and  their  strength  is  correspondingly 
greater.  It  is  usually  necessary  to  use  stirrups  with  these 
beams,  and  the  method  of  calculation  is  too  complex  to  be 
discussed  here. 

Arches  and  Hollow  Cylinders  Subjected  to  External  Pressure. 
—  The  design  of  arches  and  hollow  cylinders  subject  to  exter- 
nal pressure  is  a  complex  matter  and  beyond  the  scope  of  this 
book. 


122     CONCRETE  CONSTRUCTION  FOR  RURAL  COMMUNITIES 


PROBLEMS 

1.  Compute  the  amount  of  reinforcing  required  in  a  cylindrical  water  tank 
12  feet  in  diameter  and  8  feet  high  at  points  2,  4,  6,  and  8  feet  below  the  top. 
Select  suitable  bars  and  spacing  for  the  reinforcement  of  the  tank  from  top  to 
bottom,  so  as  to  provide  approximately  this  reinforcing.  What  thickness  of 
wall  would  you  use? 

2.  Find  the  amount  of  reinforcing  required  in  a  concrete  silo  18  feet  in 
diameter  and  40  feet  high,  at  points  10,  20,  39,  and  40  feet  from  the  top,  if 
the  pressure  of  silage  be  assumed  to  be  equivalent  to  that  of  a  liquid  weighing 
11  pounds  per  cubic  foot.    Select  suitable  bars  and  spacings  for  these  points. 

3.  Find  the  safe  load  on  a  concrete  column  16"  x  16"  x  12'  long.  What 
reinforcing  would  you  use  in  the  column?  What  would  -be  the  safe  load  on 
this  column  in  a  building  required  to  be  fire-proof? 

4.  Find  the  size  of  a  concrete  column  to  carry  a  load  of  40  tons  applied 
centrally,  if  the  length  is  12  feet.    What  reinforcing  should  be  used? 

5.  Solve  problem  4,  assuming  the  length  of  the  column  to  be  20  feet;  assum- 
ing it  to  be  5  feet. 

6.  What  is  the  safe  uniformly  distributed  load  on  a  beam  10  inches  wide 
by  16  inches  deep,  to  the  center  of  the  steel,  and  18  feet  long,  if  it  is  reinforced 
by  two  1-inch  round  rods?  Would  stirrups  be  required  in  this  beam?  What 
would  be  the  safe  load  concentrated  at  the  middle  of  the  beam? 

7.  Design  a  rectangular  reinforced  concrete  beam  to  carry  a  safe  load  of 
10,000  pounds  concentrated  at  the  middle  of  a  16-foot  span. 

8.  What  safe  load  per  square  foot  can  be  placed  on  a  floor  slab  6  inches 
thick,  reinforced  with  £-inch  square  rods  spaced  5£  inches  on  centers,  if  the 
center  of  the  steel  is  one  inch  above  the  bottom  of  the  slab  and  the  span  is 
10  feet? 

9.  Design  a  floor  slab  to  carry  a  safe  load  of  200  pounds  per  square  foot, 
on  an  8-foot  span. 

10.  What  will  be  the  cost  of  the  steel  for  the  floor  of  problem  8,  if  the  width 
is  16  feet,  and  the  steel  costs  2|  cents  per  pound? 


PART  IV 
MISCELLANEOUS    MATTERS 


CHAPTER  IX 
CONCRETE    SURFACE   FINISHES 

Methods  of  Treatment.  —  As  concrete  is  a  plastic  or  semi- 
fluid substance  when  it  is  first  placed,  it  will  show  all  the 
irregularities  of  the  forms  when  the  latter  are  removed.  Fre- 
quently, every  joint  between  the  boards,  every  imperfection  in 
the  forms,  and  even  the  grain  of  the  wood  can  be  traced  on 
the  surface  of  the  concrete.  The  honeycombed  places  which 
sometimes  appear,  even  in  excellent  work,  together  with  the 
rough  places  left  where  mortar  ran  out  into  the  cracks  between 
boards,  give  the  surface  an  unfinished  appearance.  No  special 
treatment  is  necessary  where  the  appearance  is  unimportant,  or 
where  the  surfaces  are  to  be  covered,  except  to  patch  the  open 
pockets.  This  should  be  done  as  soon  after  removing  the  forms 
as  possible,  using  a  mortar  mixed  in  about  the  same  proportions 
as  were  used  for  the  cement  and  sand  in  the  original  concrete. 
For  many  purposes  the  unfinished  appearance  of  the  surface 
is  objectionable,  and  some  kind  of  treatment  is  desirable.  The 
principal  methods  used  are: 

1.  Providing  a  rich  mortar  face,  which  may  be  troweled  or  floated 
after  the  form  is  removed. 

2.  Brushing  the  surface  with  a  cement  grout. 

3.  Scrubbing  the  surface  with  a  stiff  brush  and  water. 

4.  Rubbing  the  surface  with  a  hard  brick  and  sand,  or  a  carbo- 
rundum block. 

5.  Working  over  the  surface  with  a  cutting  tool. 

6.  Plastering  the  surface. 

Mortar  Face.  —  In  molded  articles  of  concrete,  such  as  bricks 
and  building  blocks,  it  is  common  practice  to  use  a  rich  mortar 
face,  while  the  body  of  the  block  is  made  of  a  leaner  mixture. 
This  is  easily  accomplished  by  making  the  block  face  down 
and  putting  about  an   inch  of  rich  mortar  in  the  mold   first, 

125 


126     CONCRETE  CONSTRUCTION  FOR  RURAL   COMMUNITIES 

then  using  the  leaner  mixture.  The  dry  mixture,  with  a  metal 
mold,  makes  a  fairly  satisfactory  surface,  and  usually  no 
special  treatment  is  attempted.  . 

A  mortar  face  may  also  be  used  in  walls  with  little  trouble. 
Just  before  the  concrete  is  poured,  the  surface  of  the  form  may 
be  plastered  with  a  rich  mortar.  If  this  is  mixed  to  the  right 
consistency,  it  will  adhere  to  the  forms  fairly  satisfactorily. 
The  concrete  is  then  poured,  and  worked  into  thorough  contact 
with  the  mortar  face,  care  being  taken  that  no  stones  are 
forced  through  this  face  so  as  to  show  on  the  surface  when  the 
forms  are  removed. 

Another  method  of  accomplishing  the  same  results  is  by 
means  of  a  sheet-metal  gage,  as  shown  in  Fig.  45.     This  has 


Handles  Eiveted 
to  Flared  Edge 


Fig.  45.  —  Sheet  Metal  Gage  for  Making  a  Mortar  Face 
on  a  Concrete  Wall. 

riveted  upon  its  face  two  or  more  angles,  with  the  outstanding 
legs  equal  in  width  to  the  thickness  of  the  mortar  facing  desired. 
This  is  placed  against  the  form,  the  concrete  is  poured  outside 
of  it,  and  the  mortar  is  put  between  it  and  the  form.  The 
upper  edge  of  the  sheet  is  bent  back,  as  shown  in  the  figure,  to 
make  it  easy  to  get  the  mortar  into  place.  When  the  concrete 
has  been  filled  on  the  outside,  and  mortar  on  the  inside,  to  the 
top  of  the  gage,  the  latter  is  raised  a  distance  nearly  equal  to 
its  width,  the  concrete  is  thoroughly  compacted  and  worked 
into  contact  with  the  face,  and  the  placing  of  concrete  and 
mortar  is  resumed. 

By  either  of  the  above  methods  there  is  very  little  danger  of 
the  concrete  cracking  off,  as  there  is  when  the  mortar  is  plas- 
tered   to    the    concrete    after    the    forms    have    been    removed. 


•CONCRETE  SURFACE  FINISHES 


127 


Since  both  the  facing  and  the  backing  are  placed  at  the  same 
time,  a  thoroughly  good  bond  is  secured. 

If  the  forms  are  taken  off  as  early  as  possible,  the  mortar 
face  may  be  floated  down  to  a  uniform  finish,  all  of  the  form 
marks  being  thus  removed. 


Ill  mm   Tf'f 

"""r!!iil!!)|j» 
,  Wm  'Hi      SHhI 

■  * 

Fig.  46.  —  A  Reinforced  Concrete  House. 


Brushed  Surface.  —  One  of  the  cheapest  and  most  easily 
applied  finishes  is  obtained  by  brushing  the  surfaces  with  a 
thin  coat  of  neat  cement,  or  equal  parts  of  cement  and  fine 
sand,  mixed  with  water  to  a  creamy  consistency.  A  whitewash 
brush  or  even  an  old  broom  will  answer  for  this  purpose.  The 
brushing  should  be  done  as  soon  after  the  concrete  is  poured 
as  the  forms  can  be  removed.  Unless  the  concrete  is  still  very 
green,  the  surface  should  be  thoroughly  saturated  with  water 
before  the  brush  coat  is  applied.  Surface  imperfections  should 
always  be  removed  before  the  grout  is  applied.  All  projections 
should  be  rubbed  off  and  all  pockets  should  be  patched.  This 
treatment  is  very  effective  in  filling  up  the  surface  pores  and 
helping  to  make  the  concrete  water-tight.    A  second  coat  should 


128     CONCRETE  CONSTRUCTION  FOR  RURAL  COMMUNITIES 

usually  be  applied  over  the  first  as  soon  as  the  latter  has  set 
sufficiently. 

This  finish  is  likely  to  check  if  it  is  allowed  to  dry  too  rapidly, 
or  if  it  is  exposed  to  the  direct  action  of  the  sun.  Wet  cloths 
laid  over  the  surface  or  frequent  sprinkling  during  the  first 
few  days  will  help  to  prevent  checking. 

Scrubbed  or  Etched  Surface.  —  Another  method  of  treatment 
which  is  perhaps  as  satisfactory  as  any,  considering  the  results 
secured  and  the  cost,  is  to  scrub  the  surface  of  the  concrete 
as  soon  as  the  forms  are  removed,  with  a  stiff  brush  and  water 
to  remove  the  thin  film  of  cement  outside  the  aggregate.  The 
length  of  time  which  must  elapse  after  pouring  the  concrete 
before  the  surface  can  be  scrubbed  will  vary  with  circumstances 
from  about  12  to  36  hours.  If  the  scrubbing  is  done  too  soon, 
the  aggregate  will  be  torn  out;  if  not  soon  enough,  the  cement 
film  cannot  be  removed. 

Instead  of  a  brush  one  may  use  a  carborundum  block,  or  a 
hard  brick  with  sand  for  a  cutting  agent,  water  being  used  as 
before  to  wash  away  the  material  removed.  With  the  carbo- 
rundum block  or  the  brick  and  sand,  the  scrubbing  can  be  done 
when  the  concrete  is  a  little  harder  than  it  can  be  when  the 
brush  is  used,  and  they  are  probably  to  be  preferred  to  the 
brush. 

By  using  selected  aggregates,  such  as  pebbles  or  colored  sand 
or  stones,  especially  pleasing  effects  can  be  produced. 

A  modification  of  the  above  method  is  to  apply  a  dilute  solu- 
tion of  muriatic  acid  to  the  surface.  This  will  eat  out  the 
cement,  leaving  the  aggregates  exposed.  After  the  action  has 
gone  far  enough,  the  excess  acid  should  be  thoroughly  washed 
from  the  surface.  When  limestone  has  been  used  in  the  aggre- 
gates, these  will  also  be  attacked  by  the  acid  and  the  result  is 
likely  to  be  unsatisfactory. 

Any  of  the  above  methods  of  scrubbing  or  etching  the  sur- 
face can  be  used  successfully  in  connection  with  the  mortar 
face. 

Tooled  Surface.  —  A  very  satisfactory  surface  can  be  made 
by  picking  or  cutting  the  surface  of  the  concrete  by  means  of  a 
stone  cutter's  bush  hammer,  toothed  chisel,  or  pick.     The  con- 


CONCRETE  SURFACE  FINISHES  129 

crete  should  be  allowed  to  set  two  or  three  weeks  before  this 
is  done.  If  the  work  is  attempted  earlier,  the  particles  of  ag- 
gregate near  the  surface  will  be  broken  loose,  while,  if  a  longer 
time  is  allowed,  the  concrete  will  be  unnecessarily  hard  and 
the  work  will  be  difficult. 

Plastered  Surface.  —  A  cement  plaster  is  often  used  on  the 
surface  of  concrete,  with  varying  degrees  of  success.  If  the 
work  is  properly  done,  the  results  will  often  be  entirely  satis- 
factory, but  great  care  is  required  or  the  plaster  coat  will 
eventually  crack  and  scale  off.  Generally  other  methods  of  fin- 
ishing are  to  be  preferred,  especially  by  those  not  skilled  in 
this  kind  of  work. 

Forms  for  concrete  which  is  to  be  plastered  should  not  be 
oiled,  nor  coated  with  a  soap  solution,  but  instead  should  be 
thoroughly  wet  before  the  concrete  is  placed  in  them. 

A  mortar  composed  of  one  part  of  cement  to  one  and  one- 
half  or  two  parts  of  sand  is  generally  used.  This  will  be  im- 
proved by  the  addition  of  hydrated  lime  in  any  amount  up  to 
about  one-tenth  the  volume  of  the  cement. 

For  best  results,  the  plaster  should  be  applied  while  the 
concrete  is  still  green.  The  forms  should  be  removed  as  early 
as  possible.  In  small  work  like  curbs  and  steps,  this  can  be 
done  in  a  very  short  time  if  a  medium  or  a  dry  mixture  has 
been  used.  If  the  concrete  has  dried  out,  the  surface  should 
be  roughened,  and  should  be  thoroughly  saturated  with  water 
before  plaster  is  applied,  so  that  the  water  will  not  be  absorbed 
from  the  latter  too  rapidly.  A  wash  of  neat  cement  and  water 
mixed  to  a  creamy  consistency  should  be  applied,  followed  im- 
mediately by  the  plaster. 

Best  results  will  be  obtained  if  the  plaster  coat  on  green  con- 
crete is  made  as  thin  as  possible,  preferably  not  more  than  ^V 
to  |  inch  thick.  If  a  thicker  plaster  is  necessary,  it  may  be 
placed  in  successive  coats  as  directed  for  stucco  in  the  next 
chapter.  The  surface  of  the  plaster  may  be  finished  by  any  of 
the  methods  there  given  for  stucco  finish. 

Plaster  work  on  concrete  must  be  kept  from  drying  out  too 
quickly.  Frequent  sprinkling  of  the  surface  or  protection  by 
wet  burlap  is  effective. 


130    CONCRETE  CONSTRUCTION  FOR  RURAL  COMMUNITIES 

Interior  walls  and  ceilings  of  concrete  may  be  plastered  with 
the  hard  wall  plasters  in  the  same  manner  as  brick  or  stone. 
There  is  some  danger  that  the  plaster  will  not  adhere  to  the 
concrete  on  account  of  the  smoother  surface.  Roughening  the 
surface,  and  saturating  it  with  water  before  applying  the  plas- 
ter, will  aid  materially  in  securing  a  good  bond. 


CHAPTER  X 
STUCCO    AND   PLASTER   WORK 

Uses  of  Stucco.  —  The  term  stucco  as  used  in  this  book 
refers  to  cement  mortar  used  as  an  exterior  coat  of  a  building 
or  other  structure.  The  term  is  sometimes  used  to  designate 
gypsum,  lime,  or  even  mud  plasters,  either  in  interior  or  in 
exterior  work,  but  will  not  be  so  used  here. 


Fig.  47.  —  Stucco  Garage  on  Deep  Rib  Metal  Lath. 

Good  Portland  cement  stucco  serves  as  an  excellent  outer 
coat  for  a  building,  as  it  is  cheap,  durable,  'and  pleasing  in  ap- 
pearance, if  properly  applied,  and  there  is  no  cost  for  painting  or 
repairs.  The  use  of  stucco  has  been  increasing  rapidly,  and  there 
is  every  reason  to  believe  that  it  will  continue  to  grow  in  favor. 

This  material  is  used  in  two  general  classes  of  work:  (1) 
renovating  old  buildings  of  wood,  brick,  stone,  and  concrete; 
and  (2)  new  buildings,  chiefly  on  frame,  brick,  or  tile  walls. 

Stucco  is  especially  well  adapted  to  recoating  old  houses.     It 

131 


132    CONCRETE  CONSTRUCTION  FOR  RURAL  COMMUNITIES 

may  be  applied  to  metal  lath  or  furring  strips  nailed  directly 
to  the  weather  boarding.  On  old  brick  or  stone  walls  it  is 
best  applied  over  metal  lath,  but  it  may  be  used  without  the 
lath  if  the  precautions  indicated  in  the  specifications  at  the 
end  of  this  chapter  are  observed.  It  is  not  recommended  for 
general  use  on  concrete  walls,  some  other  method  of  finishing, 
as  indicated  in  the  last  chapter,  being  more  often  desirable. 

On  new  work,  stucco  is  much  used  in  place  of  weather- 
boarding  in  frame  structures.  The  construction  is  carried  out 
exactly  the  same  as  in  the  ordinary  frame  building  as  far  as 
the  studding,  care  being  taken  that  the  latter  are  well  braced. 
The  sheathing  may  be  applied  to  the  studs  in  the  usual  manner 
and  the  lath  and  stucco  applied  outside  of  this,  or  the  lath 
and  stucco  may  be  applied  directly  to  the  studs.  Where  sheath- 
ing is  used  with  stucco,  it  is  desirable  to  have  it  run  diagonally,  so 
as  to  better  brace  the  studding  and  thus  prevent  possible  crack- 
ing of  the  stucco.  The  use  of  stucco  on  new  brick  walls  permits 
the  use  of  common  brick  throughout  the  wall,  thus  avoiding  the 
expense  and  delay  involved  in  the  use  of  facing  brick. 

Hollow  tile  is  a  material  admirably  adapted  for  use  with  a 
stucco  face.  The  stucco  may  be  applied  directly  to  the  tile 
without  lath  or  furring,  while  the  inside  of  the  tile  may  also 
be  plastered  direct  without  the  use  of  lath  or  furring,  and  with- 
out danger  of  dampness  working  through  the  wall,  The  con- 
struction is  cheap,  substantial,  dry,  sanitary,  warm  in  winter 
and  cool  in  summer,  and  fire-proof,  and  so  has  many  advan- 
tages over  frame  construction. 

Stucco  Surface  Finishes.  —  A  wide  variety  of  finishes  may  be 
had  in  stucco  work,  differing  both  in  color  and  in  the  rough- 
ness of  surface.  Variations  in  the  color  may  be  obtained  by 
selection  of  the  aggregates,  by  the  addition  of  lime  or  colored 
minerals,  or  by  the  use  of  white  cement.  These  methods  of 
coloring  are  all  discussed  in  Chapter  XII.  Methods  of  obtain- 
ing the  different  surface  finishes  are  given  in  paragraphs  31  to 
38  of  the  specifications  following. 

Specifications  for  Stucco.  —  The  following  specifications, 
adopted  by  the  American  Concrete  Institute,  give  full  direc- 
tions for  the  preparation  and  application  of  stucco. 


Fig.  48.  —  Stippled  Surface. 


Fig.  49.  —  Sand-Floated  Surface. 


Fig.  50.  —  Sand-Sprayed  Surface. 


Fig.  51.  —  Rough  Suction  Surface. 


pac^r    ■  •>  •  ^P"*—  * "  v*"T^.  VCOBK^JI 

".  ^':" '    :v     ■ 

• 

■^••■p*-  ■'■■  -^^^ 

?*•  ■*■■    ■'  ^9f-'~-  <"■-  ''^Mt.'- 

Fig.  52.  —  Splatter  Dash  Surface. 


Fig.  53.  —  Pebble  Dash  Surface. 


134    CONCRETE  CONSTRUCTION  FOR  RURAL  COMMUNITIES 

STANDARD    SPECIFICATIONS    FOR   PORTLAND    CEMENT 

STUCCO 

'  (Adopted  June,  1913) 

Paragraphs   marked    "a"    apply   only   to   back-plastered   walls   without 
sheathing. 

Paragraphs  marked  "b"  apply  only  to  walls  with  sheathing. 
All  other  paragraphs  apply  to  both  forms  of  construction. 

Materials 

1.  Cement. — The  cement  shall  meet  the  requirements  of  the  Stand- 
ard Specifications  for  Portland  Cement  of  the  American  Society  for 
Testing  Materials,  and  adopted  by  this  Institute  (Standard  No.   1). 

2.  Fine  Aggregate  shall  consist  of  sand,  crushed  stone,  or  gravel 
screenings,  graded  from  fine  to  coarse,  passing  when  dry  a  screen  hav- 
ing f-in.  diameter  holes,  shall  be  preferably  of  siliceous  materials,  clean, 
coarse,  free  from  loam,  vegetable,  or  other  deleterious  matter. 

3.  Lime.  — The  lime  shall  be  thoroughly  hydrated  either  by  the  manu- 
facturer, or  the  contractor.  If  hydrated  by  the  contractor,  it  shall  be 
slaked  in  sufficient  water  to  make  a  soft  paste  and  allowed  to  stand  at 
least  one  week  before  being  applied  to  the  wall. 

4.  Hair  or  Fiber.  —  There  shall  be  used  only  first  quality  long  cow 
hair,  free  from  foreign  matter,  or  a  long  cocoanut  fiber  well  combed  out. 

5.  Coloring  Matter. — Only  mineral  colors  shall  be  used,  but  no 
coloring  matter  which  is  affected  by  lime,  Portland  cement,  or  the 
elements  is  permissible. 

6.  Water  shall  be  clean,  free  from  oil,  acid,  strong  alkalies,  or 
vegetable  matter. 

Preparation  of  Mortar 

7.  Mixing. — The  ingredients  of  the  mortar  shall  be  thoroughly 
mixed  to  a  uniform  color,  sufficient  water  added  to  obtain  the  desired 
consistency,  and  the  mixing  shall  continue  until  the  cement  and  lime 
are  uniformly  distributed  and  the  mass  is  uniform  in  color  and 
homogeneous. 

The  hair  or  fiber  shall  be  added  during  the  process  of  wet  mixing. 

8.  Measuring  Proportions.  —  Methods  of  measurement  of  the  pro- 
portions of  the  various  ingredients,  including  the  water,  shall  be  used 
which  will  secure  separate  uniform  measurements  at  all  times.  All 
proportions  stated  are  by  volume.  A  barrel  of  cement  shall  be  assumed 
to  contain  4  cu.  ft.  Lime  when  used  shall  be  measured  in  the  form  of 
putty.    Hydrated  lime  shall  be  made  into  putty  before  being  measured. 


STUCCO  AND  PLASTER   WORK  135 

9.  Quantity.  —  There  shall  not  be  mixed  at  one  time  more  mortar 
than  will  be  used  within  one  hour.  Mortar  which  has  begun  to  stiffen 
or  take  on  its  initial  set  shall  not  be  used. 

10.  Hand  Mixing. — The  mixing  shall  be  done  on  a  water-tight  plat- 
form and  the  materials  shall  be  turned  until  they  are  homogeneous 
in  appearance  and  color. 

11.  Consistency.  —  The  materials  shall  be  mixed  so  as  to  provide 
sufficient  water  to  insure  a  proper  bonding  and  a  dense  mortar  free 
from  voids. 

12.  Retempering.  —  Retempering  mortar,  i.e.,  remixing  with  water 
after  it  has  partially  set,  shall  not  be  allowed. 

Structure 

13.  Framing. — Studs  spaced  at  12-in.  centers  wherever  possible 
shall  be  run  from  foundation  to  rafters  without  any  intervening  hori- 
zontal grain  in  the  wood.  These  studs  shall  be  tied  together  just 
below  the  floor  joists  by  6-in.  boards  which  will  be  let  into  the  studs 
on  their  inner  side,  so  as  to  be  flush  and  securely  nailed  to  them. 
These  boards  will  also  act  as  sills  for  the  floor  joists,  which  in  addi- 
tion will  be  securely  spiked  to  the  side  of  the  studs. 

14.  Bracing. — The  frame  of  the  building  shall  be  so  rigidly  con- 
structed and  braced  as  to  avoid  cracking  the  stucco. 

(a)  l  At  least  once  between  each  two  floors,  brace  between  the  stud- 
ding with  2"X3"  bridging. 

Or  (b)  »  Bracing  may  be  omitted,  as  the  sheathing  boards  act  as 
bracing. 

15.  Sheathing.  —  (a)  The  lath  is  to  be  fastened  direct  to  the  stud- 
ding and  back-plastered,  and  no  sheathing  boards  are  to  be  used. 

Or  (b)  Sheathing  boards  shall  be  not  less  than  6  in.  or  more  than  8 
in.  wide,  dressed  on  one  or  both  sides  to  a  uniform  thickness  of  f  in. 
They  shall  be  laid  diagonally  across  the  wall  studs  and  fastened  with 
two  nails  at  each  stud. 

16.  Inside  Waterproofing.  —  (a)  The  faces  of  the  studs  and  for  one 
inch  back  of  the  face  on  each  side  where  the  plaster  may  come  in  con- 
tact with  them  shall  be  thoroughly  waterproofed  with  tar  or  asphalt. 

Or  (b)  Over  the  sheathing  boards  shall  be  laid  in  horizontal  layers, 
beginning  at  the  bottom,  a  substantial  paper  well  impregnated  and 
thoroughly  waterproofed  with  tar  or  asphalt.  The  bottom  strip  shall 
lap  over  the  base  board  at  the  bottom  of  the  wall,  and  each  strip 
shall  lap  the  one  below  at  least  2  in.  The  paper  shall  lap  the  flash- 
ings at  all  openings.  When  required,  the  lower  horizontal  edge  of  each 
1  See  note  at  the  beginning  of  these  specifications. 


136     CONCRETE  CONSTRUCTION  FOR  RURAL  COMMUNITIES 

strip  shall  be  cemented  with  hot  or  liquid  tar  or  asphalt  compound, 
to  the  strip  below  and  to  the  grounds  of  flashings  at  all  openings. 
All  tacking  shall  be  within  2  in.  of  the  top  horizontal  edge,  where 
tacks  will  be  covered  by  the  lap  of  the  strip  above. 

17.  Furring. — When  furring  strips  form  an  integral  part  of  the 
metal  lath  to  be  used,  then  separate  furring  strips  as  described  in 
this  paragraph  are  to  be  omitted. 

(a)  Galvanized  or  painted  |-in.  crimped  furring  strips,  not  lighter  than 
22  gage  or  other  shape  giving  equal  results,  shall  be  fastened  direct 
to  the  studding,  using  lj-in.  X  14  gage  staples,  placed  12  in.  apart. 

Or  (b)  Fasten  ^-in.  galvanized  or  painted  crimped  furring,  not 
lighter  than  22  gage  or  other  shape  giving  equal  results  over  the 
sheathing  paper  and  directly  along  the  line  of  the  studs,  using  l|-in. 
X  14  gage  galvanized  staples,  placed  12  in.  apart.  The  same  depth 
of  furring  should  be  adhered  to  around  curved  surfaces,  and  furring 
strips  shall  be  placed  not  less  than  1|  in.  or  more  than  4  in.  on  each 
side  of  and  above  and  below  all  openings. 

18.  Preparation  of  Original  Surface.  —  All  roof  gutters  shall  be  fixed 
and  down-spout  hangers  and  all  other  fixed  supports  and  fasteners 
shall  be  put  up  before  the  plastering  is  done,  so  there  will  be  no  break 
made  in  the  plastering  where  they  are  permanently  fixed. 

Wall  copings,  balustrade  rails,  chimney  caps,  cornices,  etc.,  shall 
be  built  of  concrete,  stone,  tile,  or  metal  with  ample  overhang  drip 
grooves  or  lip,  and  water-tight  joints,  to  keep  water  from  behind  the 
plaster. 

If  wood  sills  are  used,  they  should  project  well  from  the  face  of  the 
plaster  and  have  ample  drip  groove  or  lip. 

Metal  lath  shall  be  stopped  far  enough  above  the  level  of  the  ground 
to  be  free  from  ground  moisture. 

Care  should  be  taken  to  provide  for  placing  all  trim  the  proper 
distance  from  the  studding  or  furring  to  show  its  right  projection 
after  the  plaster  is  on. 

19.  Lath. — The  lath  shall  be  not  thinner  than  24  gage,  galvanized 
or  painted,  expanded  metal  lath  weighing  not  less  than  3£  lb.  to  the 
square  yard,  or  woven  wire  lath,  galvanized  or  painted,  19  gage  2\ 
meshes  to  the  inch  with  stiffeners  at  8-in.  centers. 

20.  Application  of  Lath.  —  Place  lath  horizontally  over  the  furring 
strips,  driving  galvanized  staples  1\  in.  X  14  gage  8  in.  apart  over 
the  furring  strips  into  the  studding.  The  sheets  of  lath  shall  be  locked 
or  lapped  at  least  1  in.  and  tied  at  joints  between  studs  both  verti- 
cally and  horizontally  with   18-gage  wire. 


STUCCO  AND  PLASTER  WORK  137 

21.  Corners. — There  shall  be  6-in.  strips  of  metal  lath  bent  around 
the  corners  and  stapled  over  the  lath,  or  the  sheets  of  metal  lath  shall  be 
folded  around  the  corners  a  distance  of  at  least  3  in.  and  stapled  down 
as  applied.     Galvanized  corner  bead  may  be  applied  over  the  lath. 

22.  Insulation.  —  (a)  After  the  lath  on  the  outside  has  been  back- 
plastered,  the  air  space  may  be  divided  by  applying  heavy  building 
paper,  quilting,  felt,  or  other  suitable  insulating  material  between  the 
studs,  fastening  it  to  the  studs  by  nailing  wood  strips  over  folded  ends 
of  the  material.  This  insulation  should  be  so  fastened  as  to  clear 
the  bridging,  leaving  the  preponderance  of  the  air  space  next  to  the 
plaster.  Care  must  be  taken  to  keep  the  insulating  material  clear  of 
the  outside  plaster  and  to  make  tight  joints  against  the  wood  fram- 
ing at  the  top  and  bottom  of  the  spaces  and  against  the  bridging 
where  the  face  intercepts. 

Or  (b)  When  quilting,  felt,  or  other  insulating  material  is  to  be 
used  it  shall  be  applied  to  the  sheathing  boards  under  the  inside  water- 
proofing. 

23.  Brick,  Tile,  or  Cement  Block  Surfaces.  —  Existing  surfaces  to  be 
stuccoed  shall  have  all  loose,  friable,  or  soft  mortar  removed  from  the 
joints  to  a  depth  of  not  less  than  \  in.  All  dirt,  dust,  or  any  other  for- 
eign matter  shall  be  removed  by  means  of  a  wire  brush,  stiff  broom, 
or  other  effective  means.1  In  case  the  surface  has  been  painted,  is  oily, 
or  otherwise  in  such  condition  that  the  stucco  will  not  firmly  adhere, 
then  metal  furring  and  lathing  shall  first  be  applied. 

New  Surfaces  shall  have  ample  roughness  to  assure  a  strong  bond 
and  key  between  the  stucco  and  the  surface.  The  mortar  joints  shall 
not  be  less  than  |  in.  thick  and  the  mortar  shall  be  omitted  from  or 
raked  out  of  the  joints  for  at  least  §  in.  back  from  the  face  to  which 
the  stucco  is  applied.  Before  placing  the  scratch  coat  the  surface 
shall  be  brushed  clean  from  all  dust,  dirt,  or  loose  particles  and  thor- 
oughly wetted. 

Mortar  Coats 

24.  Plaster.  —  (a)  The  first  coat  shall  contain  not  more  than  two 
and  one-half  (2^)  parts  of  sand  to  one  (1)  part  of  Portland  cement  by 
volume.  If  lime  putty  is  added,  it  shall  not  be  in  excess  of  one-third 
(3)  of  the  volume  of  cement.  Hair  or  fiber  may  be  added  in  sufficient 
quantity  to  bond  the  mortar.2 

1  The  wall  may  be  cleaned  by  washing  with  a  solution  of  one  part  muriatic 
acid  to  five  parts  of  water.  After  this  is  done  it  must  be  thoroughly  washed 
with  water  before  applying  the  stucco.  —  Author. 

2  Use  about  one  pound  of  good  cow  hair  to  each  bag  of  cement.  —  Author. 


138     CONCRETE  CONSTRUCTION  FOR  RURAL  COMMUNITIES 

Or  (b)  The  first  coat  shall  contain  not  more  than  two  and  one-half 
(2^)  parts  of  sand  to  one  (1)  part  of  Portland  cement  by  volume.  If 
lime  putty  is  added  it  shall  not  be  in  excess  of  one-third  (|)  of  the  vol- 
ume of  cement.  No  hair,  fiber,  or  similar  material  of  any  kind  or  in 
any  quantity  shall  be  added  to  the  mortar. 

For  second  coat,  the  proportion  of  sand  to  cement  shall  not  be  greater 
than  2\  to  1  by  volume,  nor  shall  more  than  \  part  of  lime  putty  be 
added. 

For  third  coat,  the  proportion  of  sand  to  cement  shall  not  be  less 
than  2  to  1  nor  more  than  2|  to  1,  by  volume,  nor  shall  more  than 
|  part  of  lime  putty  be  added. 

25.  Application.  The  plastering  should  be  carried  on  continuously 
in  one  general  direction,  without  allowing  the  plaster  to  dry  at  the 
edge.  If  it  is  impossible  to  work  the  full  width  of  the  wall  at  one 
time,  the  joint  should  be  at  some  natural  division  of  the  surface,  such 
as  a  window  or  door. 

Metal  Lath,  (a)  The  first  coat  shall  be  applied  to  the  outside  of 
the  lath  and  pushed  through  sufficiently  to  give  a  good  key.  Over 
the  face  of  the  studs  the  plaster  shall  be  forced  well  through  the  lath 
in  order  to  fill  entirely  the  space  between  the  lath  and  the  stud.  The 
backing  coat  shall  be  applied  to  the  back  of  the  lath  and  shall  be  thor- 
oughly troweled  so  that  the  lath  shall  be  entirely  covered.  The  final 
coat  shall  be  applied  to  the  face  of  the  first  coat. 

Or  (b)  The  first  coat  shall  be  applied  to  the  lath  and  thoroughly 
pushed  through  against  the  inside  waterproofing  so  as  to  completely 
embed  the  metal  of  the  lath  on  both  sides.  Special  care  shall  be 
taken  to  fill  all  voids  around  furring  strips  and  where  laths  lap. 

Brick,  Tile,  or  Cement  Block  Surfaces. — The  first  coat  shall  be  forci- 
bly and  thoroughly  troweled  into  the  depressions  of  the  previously 
saturated  surface  so  as  to  make  a  firm  bond.  Care  shall  be  taken  to 
insure  the  complete  filling  by  the  mortar  of  all  crevices  and  pores. 

The  intermediate  and  final  coats  shall  be  applied  in  order  and  well 
troweled  on  to  insure  good  contact  with  previous  coats. 

26.  Roughing. — Soon  after  applying  and  before  the  initial  set  has 
taken  place,  the  surface  of  the  coats  which  are  to  receive  succeeding 
coats  shall  be  roughened  with  a  saw-toothed  paddle  or  other  suitable 
device. 

27.  Dampening. — Before  applying  mortar  the  surface  of  the  preced- 
ing coat  shall  be  thoroughly  wetted  to  prevent  absorption  of  water 
from  the  fresh  mortar. 

28.  Thickness  of  Coat,     (a)  The  first  coat  shall  be  at  least  §   in. 


STUCCO  AND  PLASTER  WORK  139 

thick  over  the  face  of  the  lath  and  project  through  behind  the  lath  about 
|  in.  The  backing  coat  shall  increase  the  thickness  behind  the  lath  to 
not  less  than  f  in.    The  final  coat  shall  be  not  less  than  f  in.  thick. 

Or  (b)  The  first  coat  shall  have  a  minimum  thickness  over  the  lath 
at  any  point  of  not  less  than  \  in.  The  intermediate  coat  shall  have 
a  thickness  of  not  less  than  \  in.  or  more  than  f  in.  The  final  coat 
shall  have  a  thickness  of  \  in.  when  placed  over  an  intermediate  coat, 
or  of  f  in.  when  placed  directly  on  the  scratch  coat. 

29.  Drying  Out. — The  final  coat  shall  not  be  permitted  to  dry  out 
rapidly,  and  adequate  precaution  shall  be  taken,  either  by  sprinkling 
frequently  after  the  mortar  has  set  hard  enough  to  permit  it  or  by 
hanging  wet  burlap  or  other  material  over  the  surface. 

30.  Freezing. — Stucco  should  never  be  applied  when  the  tempera- 
ture is  below  freezing. 

Finish 

31.  Smooth-troweled. — The  finishing  coat  shall  be  troweled  smooth 
with  a  metal  trowel  with  as  little  rubbing  as  possible. 

32.  Stippled. — The  finishing  coat  shall  be  troweled  smooth  with  a 
metal  trowel  with  as  little  rubbing  as  possible,  and  then  shall  be  lightly 
patted  with  a  brush  of  broom  straw  to  give  an  even,  stippled  surface. 

33.  Sand-floated.  —  The  finishing  coat,  after  being  brought  to  a 
smooth,  even  surface,  shall  be  rubbed  with  a  circular  motion  of  a  wood 
float  with  the  addition  of  a  little  sand  to  slightly  roughen  the  surface. 
This  floating  shall  be  done  when  the  mortar  has  partially  set. 

34.  Sand-sprayed. — After  the  finishing  coat  has  been  brought  to 
an  even  surface,  it  shall  be  sprayed  by  means  of  a  wide,  long  fiber 
brush  —  a  whisk  broom  does  very  well  —  dipped  into  a  creamy  mix- 
ture of  equal  parts  of  cement  and  sand,  mixed  fresh  every  30  min- 
utes and  kept  well  stirred  in  the  bucket  by  means  of  the  whisk  broom 
or  a  paddle.  This  coating  shall  be  thrown  forcibly  against  the  surface 
to  be  finished.  This  treatment  shall  be  applied  while  the  finishing  coat 
is  still  moist  and  before  it  has  attained  its  final  set,  i.e.,  within  3  to 
5  hours.  To  obtain  lighter  shades  add  hydrated  lime  of  5  to  15  per 
cent  of  the  volume  of  the  cement. 

35.  Splatter  Dash  or  Rough  Cast. — After  the  finishing  coat  has  been 
brought  to  a  smooth,  even  surface  and  before  attaining  final  set,  it 
shall  be  uniformly  coated  with  a  mixture  of  one  part  cement  and  two 
parts  of  sand  thrown  forcibly  against  it  to  produce  a  rough  surface  of 
uniform  texture  when  viewed  from  a  distance  of  20  ft.  Special  care 
shall  be  taken  to  prevent  the  rapid  drying  out  of  this  finish. 


140     CONCRETE  CONSTRUCTION  FOR  RURAL  COMMUNITIES 

36.  Pebble  Dash. — After  the  finishing  coat  has  been  brought  to  a 
smooth,  even  surface,  and  before  attaining  initial  set,  clean  round 
pebbles  or  other  material  as  selected,  not  smaller  than  \  in.  or  larger 
than  f  in.,  previously  wetted,  shall  be  thrown  forcibly  against  the 
mortar  so  as  to  embed  themselves  in  the  fresh  mortar.  They  shall 
be  distributed  uniformly  over  the  surface  of  the  final  coat  and  may 
be  pushed  back  into  the  mortar  with  a  clean  wood  trowel,  but  no 
rubbing  of  the  surface  shall  be  done  after  the  pebbles  are  embedded. 

37.  Exposed  Aggregates. — The  finishing  coat  shall  be  composed  of 
an  approved,  selected  coarse  sand,  marble  dust,  granite  dust,  or  other 
special  material,  in  the  proportion  given  for  finishing  coats,  and  within 
24  hours  after  being  applied  and  troweled  to  an  even  surface,  shall  be 
scrubbed  with  a  stiff  brush  and  water.  In  case  the  cement  is  too 
hard,  a  solution  of  one  part  hydrochloric  acid  in  four  parts  of  water  by 
volume  can  be  used  in  place  of  water.  After  the  aggregate  particles 
have  been  uniformly  exposed  by  scrubbing,  particular  care  shall  be 
taken  to  remove  all  traces  of  the  acid  by  thorough  spraying  with  a  hose. 

38.  Mortar  Colors. — When  it  is  required  that  any  of  the  above  fin- 
ishes shall  be  made  with  colored  mortar,  not  more  than  6  per  cent  of 
the  weight  of  Portland  cement  shall  be  added  to  the  mortar  in  the 
form  of  finely  ground  coloring  matter. 

A  predetermined  weight  of  color  shall  be  added  dry  to  each  batch 
of  dry  fine  aggregate  before  the  cement  is  added.  The  color  and  fine 
aggregate  shall  be  mixed  together  and  then  the  cement  and  lime  mixed 
in.  The  whole  shall  then  be  thoroughly  mixed  dry  by  shoveling  from 
one  pile  to  another  through  a  \-m.  mesh  wire  screen  until  the  entire 
batch  is  of  uniform  color.  Water  shall  then  be  added  to  bring  the 
mortar  to  a  proper  plastering  consistency. 

Machine  Stucco 

39.  Stucco  may  be  applied  by  a  machine  provided  the  results  ob- 
tained are  equal  to  those  produced  by  hand  work. 

Overcoating 

During  recent  years  there  has  come  into  vogue  a  method  of  remod- 
eling old  frame  houses.  This  overcoating,  as  it  is  called,  is  used  exten- 
sively in  all  sections  of  the  country,  and  the  following  practice  is 
recommended : 

40.  Where  a  furring  strip  is  used  so  deep  that  the  space  back  of 
the  lath  is  not  entirely  filled  with  plaster,  some  provision  must  be  made 
for  extending  the  old  window  and  door  frames  to  correspond  with  the 
increased  thickness  of  the  wall.     In  some  cases  the  plaster  is  brought 


STUCCO  AND  PLASTER   WORK  141 

over  the  old  frames  in  such  a  manner  that  a  recessed  window  or  door 
opening  is  made.  In  case  the  furring  strips  are  fastened  to  the  stud- 
ding, it  is  not  necessary  to  provide  for  extending  the  window  and 
door  frames,  as  the  new  stucco  finish  will  have  the  same  relations  as 
the  old  weather-boarding. 

41.  Preparation  of  Original  Surface.  —  If  the  weather-boarding  is  in 
poor  condition  it  should  be  removed  and  furring  strips  and  metal  lath 
applied  over  the  sheathing,  to  which  waterproof  paper  has  previously 
been  fastened.  It  may  be  advisable  also  to  tear  off  the  sheathing,  in 
which  case  the  furring  strips  can  be  fastened  direct  to  the  studding 
after  bracing  between  the  studs. 

Another  method  would  be  to  fasten  the  furring  strips  direct  over 
the  weather-boarding  over  which  the  metal  lath  is  applied. 

In  preparation  for  any  of  these  methods  the  house  should  be  gone 
over  carefully  to  determine  if  the  framework  is  well  enough  preserved 
to  justify  the  improvement. 

The  doors  should  be  looked  after,  the  studding  inspected,  partitions 
and  outside  walls  lined  up  and  brought  into  plumb. 

42.  Furring. — Fasten  galvanized  or  painted  £-in.  crimped  furring,  or 
other  shape  giving  equal  results,  vertically  over  the  original  surface, 
whichever  of  the  above  may  be  adopted. 

43.  Lathing   and    Plastering. — Follow   the  above   specifications   for 

stUCCOo 


CHAPTER  XI 
WATERPROOFING   AND    COLORING    CONCRETE 

Necessity  for  Waterproofing.  —  Unless  special  precautions  are 
taken,  concrete  is  likely  to  be  porous  to  such  an  extent  that  it 
will  permit  the  percolation  of  water  or  the  penetration  of  damp- 
ness. For  certain  purposes  for  which  concrete  is  otherwise 
admirably  adapted,  this  tendency  is  a  serious  drawback,  and 
much  thought  and  effort  have  been  devoted  to  the  development 
of  methods  for  damp-proofing  or  waterproofing.  In  some  cases 
all  that  is  necessary  is  to  reduce  the  percolation  of  water  so 
there  is  no  appreciable  leakage,  as  in  water  tanks  and  sewers, 
while  in  other  cases  it  is  necessary  to  prevent  the  penetration 
of  dampness,  as  in  residence  walls.  In  certain  cases,  heavy 
pressures  must  be  resisted,  while  in  others  all  that  is  necessary 
is  to  prevent  the  penetration  of  moisture  by  capillary  action. 
In  rural  structures  the  pressures  will  not  usually  exceed  those 
due  to  a  head  of  a  few  feet  of  water. 

Precautions  to  be  Observed  in  Water-tight  Work.  —  In  any 
structure  that  is  to  be  waterproof,  it  is  desirable  to  pour  the 
whole  mass  in  a  continuous  run  when  possible.  Unless  care  is 
taken  to  bond  the  new  concrete  to  the  old,  joints  will  occur 
between  successive  days'  work  which  will  permit  seepage.  If 
it  is  impossible  to  complete  the  mass  in  one  run,  the  precau- 
tions to  be  observed  in  making  the  joint  are  as  follows: 

(1)  When  work  is  stopped  for  the  day,  the  surface  should  be 
left  rough,  and  all  scum  should  be  removed.  Often  this  can 
best  be  accomplished  by  allowing  the  concrete  to  set  until  all 
surplus  water  has  disappeared  and  then  skimming  off  the  upper 
layer  to  the  thickness  of  half  an  inch,  or  whatever  is  necessary 
to  remove  all  scum.  The  surface  can  then  be  roughened  with 
a  trowel  or  a  stick. 

(2)  If  the  wall  is  of  sufficient  size  to  permit  it,  when  work  is 

142 


WATERPROOFING  AND  COLORING  CONCRETE  143 

stopped  angular  stones  may  be  embedded  at  intervals,  to  about 
half  their  length,  leaving  the  remainder  to  project  up  into  the 
new  work.  Iron  or  steel  rods  or  scrap  may  also  be  used  in 
this  manner. 

(3)  Just  before  new  concrete  is  deposited,  the  old  surface  should 
be  drenched  with  water  and  should  then  be  slushed  with  neat  ce- 
ment, or  equal  parts  of  neat  cement  and  sand,  mixed  with  water 
to  a  creamy  consistency.  In  case  the  scum  was  not  all  removed 
before  the  old  concrete  had  set  or  if  the  surface  was  left  smooth, 
it  should  be  picked  over  before  this  slush  coat  is  placed. 

Methods  of  Waterproofing.  *—  Three  general  principles  are 
employed  in  waterproof  structures: 

(1)  Reducing  the  porosity  of  the  concrete. 

(2)  Using  a  water-repellent  substance  in  the  concrete. 

(3)  Using  impervious  washes  or  coatings  on  the  surface. 

One  of  the  best  methods  of  producing  a  water-tight  concrete 
is  to  use  a  rich  mixture,  with  a  well  graded  aggregate.  The 
water-tightness  of  the  concrete  increases  very  rapidly  with  an 
increase  in  the  amount  of  cement  used.  All  structures  which 
are  to  be  water-tight  should  therefore  be  made  of  a  rich  mixture, 
with  proportions  ordinarily  about  1:2:4.  Proper  grading  of  the 
stone  or  gravel,  and  proportioning  of  the  sand  to  the  coarse 
aggregate  to  produce  the  greatest  density  (see  Chapter  IV)  are 
of  the  greatest  importance  in  this  connection.  The  water 
tightness  of  concrete  is  affected  to  an  even  greater  degree  than 
is  the  strength  by  proper  proportioning. 

Use  of  Hydrated  Lime.  —  Various  substances  may  be  added 
to  the  materials  ordinarily  used  in  making  concrete,  to  reduce  its 
porosity.  One  of  the  best  of  these  is  hydrated  lime.  Being  in 
the  form  of  a  very  finely  divided  powder,  it  helps  to  fill  up  the 
voids  of  the  concrete  and  so  prevent  the  percolation  of  water. 
The  lime  also  seems  to  lubricate  the  wet  concrete  so  that  the 
particles  slide  over  each  other  more  easily  and  thus  it  reduces 
the  number  of  open  pockets.  The  amount  of  lime  used  may 
be  equal  to  about  10  per  cent  of  the  weight  of  the  dry  cement 
for  1:2:4  concrete  or  15  per  cent  for  1 :  2\ :  5  or  leaner  mix- 
tures. This  amount  of  lime  will  not  reduce  the  strength,  but 
may  increase  it  slightly  in  the  leaner  mixtures. 


144     CONCRETE  CONSTRUCTION  FOR  RURAL  COMMUNITIES 

Use  of  Water-repellent  Compounds.  —  Ordinary  concrete  has 
a  capillary  attraction  for  water.  By  means  of  certain  sub- 
stances it  can  be  made  water-repellent,  much  as  a  greasy  sub- 
stance is  water-repellent.  Substances  commonly  used  are  oil, 
alum  and  soap,  and  lime  and  soap.  Either  of  these  combina- 
tions forms  an  insoluble  compound  which,  when  distributed 
through  the  mass  of  concrete,  is  effective  as  a  waterproofing. 

In  using  alum  and  soap,  dissolve  one  pound  of  alum  in  two 
gallons  of  water  and,  separately,  2\  pounds  of  soap  in  eight 
gallons  of  water.  The  two  solutions  may  then  be  mixed,  being 
stirred  frequently  to  prevent  the  compound  from  accumulating 
on  the  surface.  This  solution  should  be  used  instead  of  water 
in  mixing  the  concrete.  If  preferred,  the  alum  may  be  pulver- 
ized and  mixed  dry  with  the  cement,  the  soap  being  dissolved  in 
the  water.  This  method  of  waterproofing  decreases  the  strength 
of  the  concrete  somewhat,  perhaps  about  20  per  cent. 

If  lime  and  soap  are  used,  dissolve  a  quarter  of  a  pound  of 
soap  per  gallon  of  water  and  add  half  an  ounce  of  unslaked 
lime.  Stir  the  mixture  thoroughly  to  keep  in  suspension  the 
calcium  soap  which  is  formed.  This  solution  is  to  be  used 
instead  of  water  in  mixing  the  materials.  If  desired,  hydrated 
lime  may  be  mixed  dry  with  the  cement  instead  of  with  the 
water,  and  this  may  be  used  with  a  saturated  solution  of  soap 
in  cold  water.  A  considerable  excess  of  lime  will  do  no  harm, 
for  the  lime  alone  is  effective  waterproofing. 

Various  waterproofing  compounds  are  being  sold  under  trade 
names.  Some  of  these  are  in  the  form  of  liquids  to  be  added 
to  the  water  used  in  mixing,  others  are  powders  which  are  to  be 
mixed  with  the  cement,  and  still  others  are  sold  mixed  with 
cement  ready  for  use.  The  compositions  of  these  substances 
are  trade  secrets,  but  calcium  soap,  such  as  is  formed  by  the 
lime-soap  process  described  above,  seems  to  be  the  essential 
waterproofing  element  in  several  of  them.  Some  of  them  are 
good,  but  others  soon  lose  their  effectiveness. 

Tests  made  by  the  Office  of  Public  Roads  of  the  U.  S.  De- 
partment of  Agriculture  l  indicate  that  the  addition  of  a  mineral 

1  U.   S.   Dept.   Agr.    Bulletin    No.   230,    "  Oil-mixed    Portland   Cement 
Concrete." 


WATERPROOFING  AND  COLORING  CONCRETE  145 

oil  to  concrete  or  mortar  is  very  effective  in  rendering  it  damp- 
proof  and  waterproof  under  small  pressures,  and  that  the 
amount  of  oil  necessary  does  not  seriously  decrease  the  strength 
of  the  concrete. 

For  most  purposes  where  damp-proofing  is  required,  an 
amount  of  oil  equal  to  5  per  cent  of  the  weight  of  the  cement 
is  sufficient.  After  the  cement  and  sand  are  mixed  dry,  water 
is  added  and  the  mortar  is  mixed  to  a  uniform  mushy  consis- 
tency. The  oil  is  then  added  and  the  mass  is  remixed  until 
no  trace  of  the  oil  is  visible  on  the  surface  of  the  mortar.  The 
previously  wetted  stone  or  gravel  is  then  added  and  the  mass 
is  again  thoroughly  mixed. 

Specifications  for  the  oil,  as  given  by  the  Office  of  Public 
Roads,  are  given  below.  The  purpose  of  the  specifications  is  to 
eliminate  certain  oils  which  would  be  injurious  to  the  concrete. 

SPECIFICATIONS   FOR   OIL   TO   BE    USED   IN   OIL-CEMENT 

CONCRETE 

(1)  The  oil  shall  be  a  fluid  petroleum  product  and  shall  contain  no 
admixture  of  fatty  or  vegetable  oils. 

(2)  It  shall  have  a  specific  gravity  not  greater  than  0.945  at  a 
temperature  of  25°  C. 

(3)  It  shall  show  a  flash  point  of  not  less  than  150°  C.  by  the 
closed-cup  method. 

(4)  When  240  c.c.  of  the  oil  is  heated  in  an  Engler  viscosimeter  to 
50°  C,  and  maintained  at  that  temperature  for  at  least  three  minutes, 
the  first  100  c.c.  which  flows  out  shall  show  a  specific  viscosity  of  not 
less  than  15  nor  more  than  30. 

(5)  When  1  part  of  the  oil  is  shaken  up  with  2  parts  of  hundredth 
normal  caustic  soda,  there  shall  be  no  emulsification,  and  upon  allowing 
the  mixture  to  remain  quiet  the  two  components  shall  rapidly  separate 
in  distinct  layers. 

Surface  Coatings.  —  The  above  methods  of  waterproofing 
are  applicable  only  while  the  concrete  is  being  mixed,  and 
hence  cannot  be  used  with  existing  structures.  Methods  used 
to  render  such  structures  waterproof  consist  of  surface  applica- 
tions to  prevent  the  penetration  of  water,  either  by  rendering 
the  surface  of  the  concrete  impervious  and  water-repellent,  or 


146     CONCRETE  CONSTRUCTION  FOR  RURAL  COMMUNITIES 

by  applying  an  impervious  layer  to  the  surfaces.  In  any  case 
all  projections  should  be  removed  and  all  open  pockets  filled 
before  the  surface  coating  is  applied.  Methods  commonly 
used  are  as  follows: 

(1)  Cement  Wash.  —  The  methods  of  mixing  and  applying  the  wash 
are  explained  in  detail  in  Chapter  IX.  Before  the  first  coat  is  dry, 
a  second  coat  should  be  applied,  and  this  should  be  kept  moist  for 
several  days.  This  method  gives  a  pleasing  surface  finish  to  the  con- 
crete at  the  same  time  that  it  waterproofs  it,  but  the  wash  is  likely 
to  craze-crack  if  it  is  exposed  to  the  action  of  the  sun  and  rain,  espe- 
cially during  the  first  few  weeks  after  it  is  applied.  The  wash  is  very 
satisfactory  for  water  tanks,  silos,  sewer  tile,  and  all  similar  structures. 
It  is  most  effective  if  applied  on  the  side  of  the  concrete  exposed  to 
the  water. 

(2)  Sylvester's  Wash.  —  Sylvester's  wash  has  long  been  used  for 
waterproofing  brick  work  and  concrete  which  has  hardened  and  dried 
out.  It  consists  in  the  alternate  applications  of  alum  and  of  soap 
solutions  to  the  face  of  the  wall.  The  alum  solution  is  made  by  dis- 
solving eight  ounces  of  alum  per  gallon  of  water,  and  the  soap  solution 
by  dissolving  one  and  one-half  pounds  of  hard  soap  per  gallon  of 
water.  The  surface  should  be  clean  and  dry  so  that  the  solutions  will 
be  readily  absorbed.  The  air  temperature  should  not  be  less  than 
50°  F.  The  soap  solution  should  be  applied  boiling  hot,  while  the 
alum  solution  should  be  about  70°  F.  A  coat  of  the  soap  solution  is 
first  applied,  using  a  whitewash  or  other  convenient  brush  and  rubbing 
it  well  into  the  surface  but  taking  care  not  to  produce  a  froth.  This 
is  left  for  24  hours  or  until  the  surface  is  entirely  dry.  A  coat  of  the 
alum  solution  is  then  applied  and  allowed  to  dry  for  another  24  hours. 
This  is  followed  with  another  coat  of  soap  and  another  of  alum  at  simi- 
lar intervals.  Two  pairs  of  coats  should  be  sufficient  for  any  ordinary 
case,  though  additional  ones  may  be  applied  if  required.  The  effect 
of  this  treatment  is  to  form  an  insoluble  compound  of  calcium  soap  in 
the  outer  pores  of  the  concrete,  this  soap  filling  the  pores  and  acting 
as  a  water-repellent.  It  is  one  of  the  most  effective  treatments  which 
can  be  given  a  concrete  surface. 

(3)  Paraffine  Coating. — A  paraffine  coating  may  be  applied  either 
hot  or  cold.  In  either  case  the  surface  should  be  thoroughly  clean 
and  dry.  If  the  coating  is  applied  hot,  the  walls  to  be  treated  must 
first  be  heated.  The  melted  paraffine  wax  is  then  thoroughly  rubbed 
in.    In  the  cold  process,  the  paraffine  is  dissolved  in  a  volatile  liquid, 


WATERPROOFING  AND  COLORING  CONCRETE  147 

such  as  naphtha,  and  applied  with  a  brush.  The  volatile  liquid  evapo- 
rates and  leaves  the  surface  appearance  the  same  as  before,  but  with 
the  outer  pores  filled  with  paraffine.  At  least  two  coats  should  be 
given.  Sometimes  asphalt  dissolved  in  naphtha  is  used  in  the  same 
manner  as  the  paraffine.  The  materials  may  be  bought  ready  pre- 
pared for  use  under  various  trade  names. 

(4)  Sodium  Silicate  Coating. — Sodium  silicate,  or  water  glass,  which 
can  usually  be  purchased  at  drug  stores,  may  be  applied  to  the  surface 
of  concrete  with  a  brush,  to  waterproof  it.  Two  coats  are  usually  suffi- 
cient. The  treatment  is  more  expensive  than  the  soap  and  alum  treat- 
ment and  is  no  more  effective. 

(5)  Plaster  Coat. — A  plaster  coat  of  rich  mortar,  with  or  without 
waterproof  compounds,  may  be  used  to  render  a  concrete  wall  water- 
proof. It  is  recommended  that  a  mixture  of  one  part  of  cement,  two 
parts  of  sand,  and  one-fourth  part  of  hydrated  lime  be  used.  For 
details  of  the  method  of  application,  see  Chapter  IX. 

Bituminous  Shield.  —  An  impervious  shield  may  be  placed 
around  the  concrete  so  as  to  prevent  the  entrance  of  water.  It 
usually  consists  of  several  layers  of  tarred  paper  or  felt,  cemented 
together  and  covered  with  tar  or  asphalt.  This  shield  is  then 
usually  protected  by  another  layer  of  concrete  or  a  brick  wall. 
The  method,  while  highly  effective,  is  expensive  and  not  gen- 
erally available  for  work  of  the  character  considered  in  this 
book. 

COLORING    CONCRETE 

Materials  commonly  used  for  coloring  concrete  are: 

(1)  Colored  aggregates 

(2)  Mineral  Pigments 

(3)  Paints  or  washes  applied  to  the  surface 

Colored  Aggregates.  —  The  use  of  colored  aggregates  is  one 
of  the  most  effective  and  satisfactory  means  of  coloring  con- 
crete, as  the  colors  will  be  permanent,  and  will  not  appear 
forced.  If  sand  and  gravel,  or  broken  stone  and  screenings,  of 
the  color  desired  can  be  obtained,  and  the  surface  of  the  con- 
crete is  etched  with  acid  or  is  tooled  (see  Chapter  IX)  the  result 
will  be  very  effective.  When  a  rich  facing  is  used,  the  colored 
aggregate  is  required  in  the  facing,  only. 

Light  gray  concrete  can  be  made  by  the  use  of  white  Port- 


148     CONCRETE  CONSTRUCTION  FOR   RURAL  COMMUNITIES 

land  cement,  mixed  either  with  white  sand,  or  with  crushed 
quartz,  marble,  or  gray  limestone  screenings.  Hydrated  lime, 
to  the  amount  of  one-fourth  of  the  weight  of  cement  used,  will 
lighten  the  color  considerably. 

Mineral  Pigments.  —  Only  pure  mineral  pigments  should  be 
used,  as  others  are  likely  to  be  injurious  or  not  to  be  perma- 
nent. Different  tints  can  be  produced  by  the  use  of  different 
proportions  of  the  same  pigment,  so  that  care  should  be  taken 
to  measure  up  each  batch  accurately.  Colors  will  usually  be 
considerably  darker  while  the  concrete  is  wet  than  after  it  dries 
out,  and  the  colors  are  likely  to  grow  somewhat  lighter  with 
age.  Hence  considerably  more  pigment  should  be  used  than  is 
necessary  to  bring  the  wet  concrete  or  mortar  to  the  desired 
shade.  White  Portland  cement  used  with  the  colors  will  make 
it  possible  to  get  some  tints  not  otherwise  obtainable.  The 
following  table  may  be  used  as  a  rough  guide  to  the  materials 
and  proportions  required: 

Table  IX 
Materials  used  for  Coloring  Mortars 


Color  of  hardened 
mortar 

Mineral 

Pounds  of  color 

to  each  bag 

of  cement 

Gray 

Germantown  lamp  black 

Manganese  dioxide 

Excelsior  carbon  black 

Ultramarine  blue 

Ultramarine  green 

Iron  oxide 

Pompeian  or  English  red 

Roasted  iron  oxide  or  brown  ochre 

Yellow  ochre 

i 

Black 

12 

Black 

Blue 

3 
5 

Green 

6 

Red  

6 

Bright  red 

6 

Brown      

6 

Buff       

6 

Ordinary  lamp  black  should  not  be  used,  as  it  is  likely  to 
run  and  fade.  The  blue  and  green  colors  will  gradually  fade 
out.  The  others  should  be  practically  permanent  if  high-grade 
colors  are  obtained.  Lighter  tints  may  be  obtained  by  using 
smaller  amounts  of  the  minerals.  The  colors  should  be  mixed 
dry  with  the  cement  before  the  aggregate  is  added.     The  color- 


WATERPROOFING  AND  COLORING  CONCRETE  149 

ing  materials,  in  the  quantities  designated,  will  not  seriously  re- 
duce the  strength  of  the  concrete. 

Painting  Concrete  Surfaces.  —  It  is  not  generally  desirable 
to  paint  concrete  surfaces,  as  other  methods  of  finishing  give 
better  and  more  permanent  results.  Ordinary  oil  paints  can 
be  used  successfully,  if  the  concrete  is  allowed  to  become  thor- 
oughly dry  and  is  given  a  priming  coat  of  a  solution  of  eight 
pounds  of  zinc  sulphate  per  gallon  of  water.  Various  prepared 
paints  which  do  not  use  oil  as  a  base,  and  which  serve  also  as 
waterproof  coatings,  can  be  purchased.  A  wide  range  of  colors 
is  available. 


CHAPTER  XII 
CASTING   IN   MOLDS 

Separately  molded  concrete  units  are  widely  used  in  the 
forms  of  building  bricks  and  blocks,  including  sills,  lintels,  cor- 
nice blocks,  columns,  and  other  building  elements;  of  fence  and 
other  posts;    and  of  hollow  tile  for  drains  and  sewers. 

When  they  are  properly  made  and  used,  cast  concrete  speci- 
mens are  entirely  satisfactory  for  the  above  purposes,  and  have 
much  to  recommend  them.  Many  unsatisfactory  blocks,  posts, 
and  tile  have  been  made  and  sold,  either  from  ignorance  of 
proper  methods  of  manufacture,  or  from  a  desire  to  cheapen 
the  product.  The  unsuccessful  attempts  to  imitate  stone 
through  the  use  of  the  so-called  "rock  face"  and  of  artificial 
coloring  have  helped  to  give  to  concrete  blocks  the  impression 
of  cheapness,  and  to  bring  them  into  disrepute.  As  proper 
methods  of  manufacture  and  use  become  more  generally  em- 
ployed, there  will  undoubtedly  be  a  great  growth  in  the  use  of 
these  materials. 

Building  Blocks.  —  The  advantages  claimed  for  building 
blocks  are  as  follows: 

'  1.  They  are  often  much  cheaper  in  first  cost  than  brick  or  stone. 

2.  They  can  be  manufactured  near  the  building  site,  thus  saving 
transportation  charges. 

3.  They  can  be  more  cheaply  laid  into  the  wall  than  brick,  because 
of  the  larger  size  of  the  blocks. 

4.  On  account  of  fewer  joints  than  in  brick  work,  a  considerable 
saving  in  mortar  results. 

5.  The  air  spaces  in  the  walls  make  the  building  cool  in  summer 
and  warm  in  winter. 

6.  The  air  spaces  help  to  prevent  water  from  soaking  through  the 
wall  so  that  with  well-made  blocks  the  plastering  may  be  done  directly 
on  the  inner  face  of  the  wall,  without  furring. 

150 


CASTING  IN  MOLDS  151 

7.  The  air  spaces  permit  pipes  and  wires  to  be  concealed  in  the 
walls. 

8.  The  construction  is  substantial  and  fire-proof. 

Processes  of  Manufacture.  —  Concrete  blocks  may  be  made 
by  either  the  wet  process  or  the  dry  process. 

In  the  wet  process  enough  water  is  added  to  the  mixture  to 
make  a  mushy  consistency,  similar  to  that  used  in  ordinary 
poured  concrete  work.  The  blocks  must  be  left  in  the  molds 
until  they  harden,  and  hence  many  molds  are  required.  For 
this  reason  this  method  is  little  used,  notwithstanding  the  fact 
that  it  gives  better  blocks  than  the  dry  process  when  the 
same  proportions  are  used. 

In  the  dry  process  only  enough  water  is  added  to  make  the 
concrete  of  the  consistency  of  damp  earth.  This  is  heavily 
rammed  or  compressed  into  the  mold,  which  can  then  be 
removed  immediately,  the  concrete  retaining  its  shape.  By 
this  means  but  one  mold  is  required  to  carry  on  the  work 
continuously.  As  much  water  should  be  used  as  is  possible 
without  causing  the  block  to  fail  when  the  mold  is  removed. 
Otherwise  the  blocks  will  be  weak  and  porous. 

Block  Machines.  —  Many  concrete  block  machines  ai«e  on  the 
market  designed  to  facilitate  (1)  the  handling  of  the  blocks, 
(2)  the  use  of  cores  for  making  the  blocks  hollow,  and  (3)  the 
use  of  a  rich  mortar  for  the  faces  of  the  blocks.  They  may  be 
classified  as: 

(1)  Vertical  face,  or  upright,  machines, 

(2)  Face-down  machines, 

(3)  Machines  for  making  two-piece  blocks, 

(4)  Machines  for  use  with  wet  concrete. 

The  first  three  types  use  the  dry  process.  In  the  first,  the 
mold  consists  of  a  wooden  or  metal  rectangular  frame,  so  hinged 
and  fastened  that  the  block  can  readily  be  released  after  it  is 
made.  Upright  cores  are  used  for  making  the  blocks  hollow. 
If  it  is  desired  to  use  a  richer  face  than  body,  or  one  of  a  dif- 
ferent color,  a  vertical  parting  plate  is  used.  This  is  inconven- 
ient, and  care  must  be  taken  to  get  a  satisfactory  bond  between 
the  face  and  the  body  of  the  block.     For  this  reason  these 


152     CONCRETE  CONSTRUCTION  FOR  RURAL  COMMUNITIES 


Fig.  54.  —  Vertical  Face  Mold  for  Column 
Blocks. 


machines  are  much  less  satisfactory  for  faced  blocks  than  are 
the  face-down  machines.  Wooden  or  cast-iron  bottom  plates 
are  used,  some  machines  being  designed  to  be  lifted  bodily  from 

the  block  and  others  being 
stationary  so  that  the 
blocks  are  lifted  from  the 
machine  on  their  bottom 
plates.     (See  Fig.  54.) 

In  the  second  class,  or 
face-down  machines,  the 
face  plate  forms  the  bot- 
tom of  the  mold,  the  bot- 
tom board  forms  one  side, 
and  the  cores  are  horizon- 
tal. If  the  blocks  are  to 
have  a  special  facing  ma- 
terial, this  is  thrown  into  the  mold  first  and  leveled  off.  The 
body  mixture  is  added  to  the  height  of  the  core,  and  tamped. 
The  core  is  then  put  into  place,  and  the  filling  and  the  tamping 
are  continued.  The  machine  is  then  rolled  over  so  that  the  bot- 
tom plate  is  below  the  block,  the  cores  and  sides  of  the  mold  are 
released,  and  the  block  is  removed  on  its  bottom  plate.  Machines 
of  this  type  are  very  widely  used.  Figure  55  shows  one  of  them. 
Machines  of  the  third  class  are  built  to  manufacture  blocks 
nearly  automatically,  the  shape  of  the  blocks  being  such  that 
they  can  be  compacted  by  pressing  instead  of  by  tamping. 
The  machines  are  large  and  complicated  and  are  designed  for  a 
large  output.  A  wall  of  two-piece  blocks  is  shown  in  Fig.  56. 
The  mixture  is  shoveled  or  dropped  into  the  mold,  pressure  is 
applied,  and  the  block  is  at  once  released.  If  facing  is  desired, 
this  is  applied  in  the  top  of  the  mold  before  pressing.  The 
pressure  being  applied  directly  to  the  face,  the  latter  is  made 
very  dense  and  hard  and  a  good  bond  is  obtained  with  the 
backing. 

In  the  fourth  class  of  machines,  a  large  number  of  wooden  or 
metal  forms  and  cores  are  provided  of  the  shape  and  size  de- 
sired for  the  blocks,  and  these  are  filled  with  a  mushy  concrete. 
After  twelve  to  twenty-four  hours,  the  side  forms  and  the  cores 


CASTING  IN  MOLDS 


153 


Fig.  55.  —  Face-down  Block  Machine. 


can  be  removed  and  again  used.     The  blocks  are  likely  to  be  less 
porous  and  stronger  than  those  made  by  the  dry  process. 

Kinds  of  Blocks.  —  A  large  variety  of  shapes  and  sizes  is 
used  for  the  blocks,  and  for 
the  cored  spaces.  Common 
outside  dimensions  are  12,  16, 
20,  or  24  inches  long,  4,  8,  or 
12  inches  high,  and  8,  10,  or 
12  inches  thick.  The  larger 
sizes  are  rather  heavy,  so  that 
the  16-inch  blocks  8  inches 
high  are  being  much  used. 
The  use  of  various  sizes  in  the 
same  wall  avoids  the  monoto- 
nous effect    which   sometimes 


results  when  all  are  of  the 


Fig.  56.  —  Wall  of  Two-piece  Concrete 
Blocks. 


154     CONCRETE  CONSTRUCTION  FOR  RURAL  COMMUNITIES 

same  size.     (See  Fig.  57.)     Curved  blocks  are  made  for  use  in 
silos  and  other  round  structures. 


Fig.  57.  —  Broken  Ashlar  Effect  from  Concrete  Blocks. 


The  arrangement  of  cores  should  be  such  as  to  give  the  block 
sufficient  strength  and  still  make  it  light,  and  prevent  the  pas- 
sage of  moisture  to  the  inside  of  the  wall.  Staggered  air  spaces 
are  very  effective  in  preventing  the  passage  of  moisture,  but 
the  blocks  are  likely  to  be  weaker  than  those  with  webs  extend- 
ing through  the  block. 

Waterproofing  and  Coloring  of  Blocks.  — The  same  methods 
can  be  used  for  waterproofing  and  coloring  blocks  as  for  other 
forms  of  concrete.  These  are  discussed  elsewhere  in  this  book. 
An  impervious  partition  may  be  formed  in  the  blocks  made  by 
the  face-down  machine  by  placing  a  thin  layer  of  rich  mortar 
in  each  of  the  web  spaces  after  the  cores  have  been  put  into 
place.  In  this  way,  but  little  of  the  rich  mixture  is  required,  and 
moisture  is  effectively  prevented  from  passing  through  the  wall. 

Curing  of  Blocks.  —  Since  blocks  are  usually  made  with  but 
little  water,  it  is  especially  important  that  they  should  not  be 
exposed  to  wind  or  sun  and  that  they  should  be  frequently 
sprinkled.  The  first  sprinkling  should  be  given  as  soon  as  is 
possible  without  washing  out  the  cement,  and  the  blocks  should 
be  kept  moist  for  at  least  a  week.  They  should  be  left  on  their 
bottom  boards  for  forty-eight  hours  or  until  they  can  be  handled 
without  injury,  and  should  be  allowed  to  cure  for  about  a  month 
before  being  built  into  walls.  Some  manufacturers  cure  their 
blocks  in  a  closed  room  which  is  kept  moist  and  heated  to  a 


CASTING  IN  MOLDS  155 

high  temperature  by  the  use  of  steam.  Blocks  cured  in  this 
manner  will  become  as  strong  in  twenty-four  hours  as  they 
would  in  several  days  at  ordinary  temperatures. 

Cost  of  Concrete  Blocks.  —  Concrete  block  walls  will,  where 
conditions  are  favorable,  be  considerably  cheaper  than  either 
brick  or  stone.  For  one  hundred  blocks  8"  X  8"  X  16",  of  1 :  3:  4 
concrete,  with  §  inch  of  1 : 2  facing,  allowing  J  of  the  volume  of 
the  blocks  for  cores,  the  cost  may  be  estimated  as  follows: 

8f  bags  cement  @  $0.40 =  $3.40 

i  cu.  yds.  sand  @  $0.75 -    0.65 

1  cu.  yd.  gravel  @  $1.25       =    1.25 

Total  cost  of  materials =  $5.30 

Labor,  2  men,  6  hrs.  @  20^  per  hour =  $2.40 

Total  cost  of  100  blocks  at  factory -  $7.70 

Allowing  50  per  cent  for  profit  and  incidental  expenses,  the 
blocks  could  be  sold  for  about  12  cents  each  at  the  factory. 
Allowing  3  cents  each  for  hauling  and  7  cents  for  laying  will 
make  the  total  cost  per  block  in  the  wall  22  cents,  or  about  25 
cents  per  square  foot  of  wall  face.  The  prices  given  above  will 
vary  in  different  localities  and  at  different  times,  but  may  be 
taken  as  a  fair  average. 

The  following  specifications  may  be  used  as  a  guide  to  the 
construction  and  use  of  first-class  concrete  building  blocks, 
suitable  for  any  walls  where  brick  or  stone  might  be  used. 

RECOMMENDED    PRACTICE    FOR    CONCRETE    ARCHITEC- 
TURAL   STONE,    BUILDING    BLOCKS,    AND    BRICKS 
(Adopted  by  National  Association  of  Cement  Users,  May,  1912.) 
General 

1.  This  Recommended  Practice  is  intended  to  cover  the  general 
requirements  for  the  manufacture  and  testing  of  concrete  architec- 
tural stone,  building  blocks,  and  brick. 

Materials 

2.  Cement.  —  The  cement  shall  meet  the  requirements  of  the  Stand- 
ard Specifications  for  Portland  cement  of  the  American  Society  for 
Testing  Materials,  and  adopted  by  this  Association.     (Standard  No.  1.) 

3.  Aggregates.  —  The  aggregates  shall  be  clean,  coarse,  hard,  dur- 
able materials,  and  shall  be  free  from  dust,  soft,  flat  or  elongated  par- 


156     CONCRETE  CONSTRUCTION  FOR  RURAL  COMMUNITIES 

tides,  loam,  vegetable  or  other  deleterious  matter.     In  no  case  shall 
aggregate  containing  frost  or  lumps  of  frozen  material  be  used. 

(a)  Fine  Aggregate.  —  The  fine  aggregate  shall  consist  of  sand,  crushed 
stone  or  gravel  screenings,  preferably  of  siliceous  material,  graded  from 
fine  to  coarse,  and  passing,  when  dry,  a  screen  having  one-quarter  (|) 
inch  diameter  holes;  not  more  than  twenty  (20)  per  cent  shall  pass  a 
sieve  having  fifty  (50)  meshes  per  linear  inch,  and  not  more  than  six  (6) 
per  cent  pass  a  sieve  having  one  hundred  (100)  meshes  per  linear  inch. 

Fine  aggregate  shall  be  of  such  quality  that  mortar  composed  of 
one  (1)  part  Portland  cement  and  three  (3)  parts  fine  aggregate  by 
weight  when  made  into  briquettes  will  show  a  tensile  strength  at  least 
equal  to  the  strength  of  1 : 3  mortar  of  the  same  consistency  made 
with  the  same  cement  and  Standard  Ottawa  sand. 

(b)  Coarse  Aggregate.  —  The  coarse  aggregate  shall  consist  of  gravel, 
crushed  stone,  or  other  suitable  material  graded  in  size,  which  is  re- 
tained on  a  screen  having  one-quarter  (j)  inch  diameter  holes.  In  no 
case  shall  the  maximum  dimension  be  greater  than  one-half  the  mini- 
mum width  of  any  section  of  the  finished  product. 

4.  Coloring  Matter. — Where  color  is  required,  only  the  most  per- 
manent and  durable  mineral  colors  shall  be  used  and  shall  be  consid- 
ered as  aggregate  in  measuring  proportions. 

5.  Water.  —  The  water  shall  be  clean,  free  from  oil,  acid,  strong 
alkalies  or  vegetable  matter. 

Proportions 

6.  Proportions. —  The  proportions  of  cement  to  aggregate  shall  be 
such  as  require  at  least  the  minimum  amount  of  cement  to  produce 
the  strength  and  density  specified  in  the  Standard  Specifications  for 
Concrete  Architectural  Stone,  Building  Blocks,  and  Bricks.1  The  pro- 
portions of  the  various  sizes  of  aggregates  and  cement  to  aggregates 
shall  preferably  be  made  by  weight.  If  by  volume,  a  bag  of  Portland 
cement  shall  be  considered  one  (1)  cubic  foot. 

7.  Measuring  Proportions.  —  Methods  of  measurement  of  the  pro- 
portions of  the  various  ingredients  shall  be  used  which  will  secure 
uniform  measurements  at  all  times. 

Mixing 

8.  Mixing. — The  ingredients  of  concrete  shall  be  thoroughly  mixed 
dry,  sufficient  water  added  to  obtain  the  desired  consistency,  and  the 
mixing  shall  continue  until  the  cement  is  uniformly  distributed  and 

1  Proportions  of  1 :  2\ :  4,  or  1 :  3:  5  may  generally  be  used  for  the  body,  and 
1:2  for  the  face.  —  Author. 


CASTING  IN  MOLDS  157 

the  mass  is  uniform  in  color  and  homogeneous.  The  mixing  shall 
preferably  be  done  with  a  machine  mixer  of  a  type  which  insures  the 
proper  mixing  of  the  materials  throughout  the  mass. 

9.  Consistency.  —  (a)  Wet  Process.  The  concrete  must  have  at  least 
a  sufficient  amount  of  water  to  make  it  so  soft  that  it  must  be  handled 
quickly  to  prevent  it  running  off  the  shovel,  but  not  so  thin  as  to  cause 
segregation  of  the  materials. 

(6)  Semi-wet  Process.  —  The  material  shall  be  mixed  with  a  maximum 
amount  of  water  permissible,  and  must  have  sufficient  water  so  that 
the  mixture  will  hold  its  form  after  being  compressed  in  the  hand. 

10.  Retempering.  —  Retempering,  that  is  remixing  mortar  or  concrete 
partially  hardened  with  additional  water,  or  using  mortar  or  concrete 
forty  minutes  after  being  mixed,  shall  not  be  permitted. 
Reinforcement 

11.  Reinforcement.  —  All  lintels,  bearing  stones,  and  other  members 
subjected  to  cross  bending  shall  be  reinforced  by  means  of  rods  placed 
about  one  and  one-half  inches  from  their  tension  surface,  and  the  total 
sectional  area  for  the  reinforcement  shall  not  be  less  than  0.8  of  1 
per  cent  of  the  cross-sectional  area  of  the  concrete  in  the  member 
reinforced.  When  any  member  exceeds  in  any  dimension  eight  times 
its  least  dimension,  it  shall  be  reinforced  to  insure  safety  in  handling. 

Curing 

12.  Natural  Curing. — The  concrete  products  shall  be  protected 
from  the  sun  and  strong  currents  of  air  for  a  period  of  at  least  7  days. 
During  this  period  they  shall  be  sprinkled  at  such  intervals  as  is  nec- 
essary to  prevent  drying,  and  maintained  at  a  temperature  of  not 
less  than  50°  F.  Such  other  precautions  shall  be  taken  as  to  enable 
the  hardening  to  take  place  under  the  most  favorable  conditions. 
After  7  days  the  products  may  be  removed  to  the  yard,  but  in  no  case 
used  before  they  are  21  days  old. 

13.  Steam  Curing. — The  products  shall  be  removed  from  the  molds 
as  soon  as  conditions  will  permit  and  shall  be  placed  in  a  steam-curing 
chamber  containing  an  atmosphere  of  steam  saturated  with  moisture 
for  a  period  of  at  least  48  hours.  The  curing  chamber  shall  be  main- 
tained at  a  temperature  between  100°  and  130°  F.  The  products 
shall  then  be  removed  and  stored  for  at  least  8  days.  (This  does  not 
apply  to  high  pressure  steam  curing.) 

Finishing,  Marking,  and  Handling 

14.  Finish.  Concrete  products  may  have  exposed  surfaces  treated 
by  any  of  the  various  methods  proposed  by  this  Association  in  the 


158     CONCRETE  CONSTRUCTION  FOR  RURAL   COMMUNITIES 

Report  on  Treatment  of  Concrete  Surfaces.1     All  surfaces  and  arrises 
of  stone  must  be  true  and  without  imperfections. 

15.  Marking. — All  concrete  products  of  full  standard  size  shall  be 
marked  for  purpose  of  identification,  showing  name  of  manufacturer  or 
brand,  and  date  (day,  month,  and  year)  made. 

16.  Handling. — All  concrete  products  shall  be  handled  with  utmost 
care.  When  transported  and  subjected  to  rough  handling  they  shall  be 
crated  and  packed  in  non-staining  material  in  such  a  way  as  to  insure  no 
damage  from  chipping  or  abrasion.  All  large  and  heavy  stone  shall  be 
provided  with  hooks  for  lifting.  When  necessary,  stone  shall  be  provided 
with  metal  bonds  for  the  purpose  of  tying  to  the  masonry  backing. 


PROPOSED  STANDARD  BUILDING  REGULATIONS  FOR  THE 

USE  OF  CONCRETE  ARCHITECTURAL  STONE,  BUILDING 

BLOCKS,  AND  BRICK 

(Adopted  by  National  Association  of  Cement  Users,  May,  1912.) 

I.  General 

1.  Class  of  Buildings.  — Concrete  Architectural  Stone,  Building  Blocks, 
and  Brick  meeting  the  requirements  set  forth  in  the  Standard  speci- 
fications and  Standard  Recommended  Practice  may  be  used  in  build- 
ing construction,  subject  to  the  usual  form  of  approval  required  of 
other  materials  of  construction  by  the  Bureau  of  Building  Inspection. 

2.  Height  of  Buildings.  —  The  height  of  buildings  constructed  of 
concrete  building  products  shall  be  limited  by  the  requirements  in  these 
regulations. 

II.  Details  of  Construction 

3.  Thickness  of  Walls.  —  (a)  Bearing  Walls,  25  ft.  Span.  The  thickness 
of  bearing  walls  in  such  buildings  as  garages,  stables,  office  buildings, 
hotels,  tenements,  boarding  and  lodging  houses,  and  residences  shall  be 
as  given  in  the  table  below,  for  buildings  in  which  the  maximum  dis- 
tance between  bearing  walls  or  columns  does  not  exceed  25  feet. 


No.  of 

Thickness  of  wall  in  inches 

stories 

Basement 

1st  story 

2d  story 

3d  story 

4th  story 

1 
2 
3 

4 

8 
10 
12 
16 

8 

8 

10 

12 

'8 

8 

10 

8 

8 

8 

1  See  Chapter  IX  for  methods  of  finishing  surfaces. 


CASTING  IN  MOLDS 


159 


160    CONCRETE  CONSTRUCTION  FOR  RURAL  COMMUNITIES 

(b)  Bearing  Walls,  More  than  25  ft.  Span. — In  buildings  not  covered 
by  the  above,  the  thicknesses  of  the  bearing  walls  shall  be  determined 
according  to  the  limit  of  loading  specified  in  paragraph  6.  In  no  case, 
however,  shall  any  outside  bearing  wall  be  less  than  eight  (8)  inches 
thick. 

(c)  Party  Walls. — Hollow  concrete  blocks  used  in  the  construction 
of  party  walls  shall  be  filled  solid  with  concrete  placed  on  the  job. 

(d)  Curtain  or  Partition  Walls. — For  curtain  walls  or  partition  walls, 
the  requirements  shall  be  the  same  as  in  the  use  of  hollow  tile,  terra 
cotta,  or  plaster  blocks. 

4.  Walls,  Laying,  etc.  —  (a)  Bonding.  Where  the  face  only  is  of 
hollow  cement  block,  and  the  backing  is  of  brick,  the  facing  of  hollow 
block  must  be  strongly  bonded  to  the  brick,  either  with  headers  pro- 
jecting four  (4)  inches  into  the  brick  work,  every  fourth  course  being 
a  header  course,  or  with  approved  ties,  no  brick  backing  to  be  less 
than  eight  (8)  inches  thick.  Where  the  walls  are  made  entirely  of  con- 
crete blocks,  but  where  said  blocks  have  not  the  same  width  as  the 
wall,  every  fifth  course  shall  extend  through  the  wall,  forming  a  secure 
bond,  when  not  otherwise  sufficiently  bonded. 

(b)  Portland  Cement  Mortar  shall  be  made  of  Portland  cement  and 
sand  in  the  proportions  of  one  (1)  part  cement  and  not  more  than  two 
and  one-half  (2|)  parts  sand,  and  shall  be  used  immediately  after 
being  mixed. 

(c)  Portland  Cement  and  Lime  Mortar  shall  be  made  of  Portland 
cement  and  sand  in  the  proportions  of  one  (1)  part  cement,  not  more 
than  two  and  one-half  (2^)  parts  sand  and  not  more  than  one-quarter 
(I)  part  hydrated  or  thoroughly  slaked  lime. 

5.  Girders  or  Joists.  —  Wherever  girders  or  joists  rest  upon  walls  so 
that  there  is  a  concentrated  load  on  the  block  of  over  two  (2)  tons, 
the  blocks  supporting  the  girder  or  joists  must  be  made  solid  for  at 
least  eight  (8)  inches  from  the  inside  face.  Where  such  concentrated 
load  exceeds  five  (5)  tons,  the  blocks  for  at  least  three  courses  below, 
and  for  a  distance  extending  at  least  eighteen  (18)  inches  each  side  of 
each  girder  shall  be  made  solid  for  at  least  eight  (8)  inches  from  the 
inside  face.  Wherever  walls  are  decreased  in  thickness,  the  top  course 
of  the  thicker  wall  shall  afford  a  full  solid  bearing  for  the  webs  and 
walls  of  the  course  of  blocks  above. 

6.  Limit  of  Loading. — No  wall  composed  of  hollow  concrete  block 
when  laid  up  in  a  Portland  cement  and  lime  mortar  shall  be  loaded  at 
any  point  to  an  excess  of  167  lb.  per  sq.  in.,  equivalent  to  twelve  (12) 
tons  per  square  foot  of  the  superficial  areas  of  such  blocks  as  used  in 


CASTING  IN  MOLDS 


161 


the  wall,  including  the  weight  of  the  wall.  When  the  blocks  are  laid 
up  in  a  Portland  cement  mortar,  this  limit  of  loading  may  be  increased 
to  200  lb.  per  sq.  in.  In  buildings  where  most  of  the  floor  loads,  etc., 
are  carried  by  pilasters,  said  pilasters  may  be  made  of  hollow  concrete 
building  blocks  and  the  air  spaces  filled  in  solid  with  slush  concrete 
placed  on  the  job.  Such  pilasters  shall  not  be  loaded  to  exceed  300  lb. 
per  sq.  in.  of  gross  cross-sectional  area. 


Fig.  59.  —  A  Round  Concrete  Block  Barn.     Note  the  Silo  in  the  Center. 


7.  Strength  of  Blocks.  — No  blocks  shall  be  used  in  bearing  walls  that 
have  a  crushing  strength  of  less  than  1000  lb.  per  sq.  in.  of  gross  cross- 
sectional  area  at  the  age  of  28  days. 

8.  Hollow  Space.  —  The  hollow  space  in  building  blocks  used  in  bear- 
ing walls  shall  not  exceed  33  per  cent  except  where,  blocks  containing 
a  greater  percentage  shall  be  proved  by  actual  tests  to  meet  all  the 
test  requirements  herein  specified  to  the  satisfaction  of  the  Bureau  of 
Building  Inspection. 


162    CONCRETE  CONSTRUCTION  FOR  RURAL  COMMUNITIES 

PROPOSED    STANDARD    SPECIFICATIONS    FOR    CONCRETE 
ARCHITECTURAL  STONE,  BUILDING  BLOCKS,  AND  BRICK 

(Adopted  by  National  Association  of  Cement  Users,  May,  1912.) 

I.  Test  Requirements 

1.  Concrete  architectural  stone,  building  blocks,  and  brick  must  be 
subjected  to  compression  and  absorption  tests.  The  test  samples  must 
represent  the  ordinary  commercial  product  of  the  regular  size  and  shape 
used  in  construction 

2.  Compression.  —  (a)  Solid  Concrete  Stone,  Block,  and  Brick.  In  the 
case  of  solid  concrete  stone,  block,  and  brick  the  ultimate  compressive 
strength  at  28  days  must  average  1500  lb.  per  sq.  in.  of  gross 
cross-sectional  area  of  the  stone  as  used  in  the  wall,  and  must  not  fall 
below  1050  lb.  per  sq.  in.  in  any  case. 

(6)  Hollow  and  Two- Piece  Building  Blocks. — The  ultimate  com- 
pressive -strength  of  hollow  and  two-piece  building  blocks  at  28  days 
must  average  1000  lb.  per  sq.  in.  of  gross  cross-sectional  area  of  the 
block  as  used  in  the  wall,  and  must  not  fall  below  700  lb.  per  sq.  in. 
in  any  case. 

(c)  Area  of  Hollow  Blocks.  —  In  the  case  of  hollow  building  blocks 
the  gross  cross-sectional  area  shall  be  considered  as  the  actual  wall 
area  including  the  block  and  air  space  displaced  by  the  block. 

(d)  Area  of  Two-Piece  Blocks.  —  In  the  case  of  two-piece  blocks  the 
blocks  shall  be  tested  in  pairs  consisting  of  the  front  and  rear  blocks 
as  used  in  the  wall.  The  compressive  strength  shall  be  regarded  as 
the  sum  total  sustained  by  the  two  blocks  divided  by  the  product  of  the 
length  of  the  blocks  and  the  width  of  the  wall. 

3.  Absorption.  —  The  percentage  of  absorption  at  28  days  (being  the 
weight  of  the  water  absorbed,  divided  by  the  weight  of  the  dry  sample) 
must  not  exceed  five  (5)  per  cent  when  tested,  as  hereinafter  specified. 

II.  Standard  Methods  of  Testing 

4.  General.  —  (a)  Laboratory.  All  tests  required  for  approval  shall 
be  made  in  some  laboratory  of  recognized  standing. 

(b)  Samples.  For  the  purpose  of  the  tests  at  least  nine  samples  or 
test  pieces  must  be  provided.  Such  samples  must  represent  the  ordi- 
nary commercial  product,  and  shall  be  selected  from  stock.  In  cases 
where  the  material  is  made  and  used  in  special  shapes  or  forms  too 
large  for  testing  in  the  ordinary  machines,  smaller  size  specimens  shall 
be  used  as  may  be  directed. 


CASTING  IN  MOLDS  163 

(c)  Tests.  Tests  shall  be  made  in  series  of  at  least  three.  The  re- 
maining samples  are  kept  in  reserve  in  case  duplicate  or  confirmatory 
tests  be  required.  All  samples  must  be  marked  for  identification  and 
comparison. 

5.  Compression  Tests. — The  compression  tests  shall  be  made  as 
follows : 

(a)  Solid  Concrete  Stone,  Block,  and  Brick.  —  When  testing  solid  con- 
crete stone,  block,  and  brick,  the  net  area  shall  be  considered  as  the 
minimum  area  in  compression. 

(b)  Hollow  and  Two-piece  Building  Blocks.  Whenever  possible  such 
tests  shall  be  made  on  full-size  blocks.  When  such  tests  must  be  made 
on  portions  of  blocks,  both  pieces  of  the  block  must  be  tested  and  both 
must  contain  at  least  one  full  web  section.  The  samples  must  be  care- 
fully measured,  then  bedded  flatwise  in  plaster  of  Paris  to  secure  uni- 
form bearing  in  the  testing  machine  and  crushed. 

The  net  area  shall  be  regarded  as  the  smallest  bearing  area  of  the 
piece  being  tested.  The  total  compressive  strength  shall  be  divided  by 
the  net  area  to  obtain  the  compressive  strength  in  lb.  per  sq.  in.  of 
net  area  of  each  piece.  The  sum  of  the  two  results  shall  then  be 
averaged  to  obtain  the  average  strength  in  lb.  per  sq.  in.  of  the  net 
area  of  the  total  block. 

The  entire  block  shall  be  carefully  measured  to  determine  the  maxi- 
mum air  space  prior  to  breaking  the  block  for  the  compressing  tests, 
and  the  net  compressive  strength  obtained  shall  then  be  reduced  to 
compressive  strength  in  lb.  per  sq.  in.  of  gross  area,  this  being  figured 
from  the  actual  air  space  of  the  block  determined  above. 

6.  Absorption  Tests.  The  sample  is  first  thoroughly  dried  to  a  con- 
stant weight  at  not  to  exceed  212°  F.  and  the  weight  carefully  re- 
corded. When  dried,  the  sample  is  to  be  immersed  in  a  pan  or  tray 
of  water  to  a  depth  of  2  in.,  resting  on  two  strips  not  over  1  in.  in 
width  to  allow  the  water  to  have  free  access  to  face.  At  the  end  of 
48  hours  from  the  time  it  is  placed  in  water,  the  sample  shall  be  re- 
moved, the  surface  water  wiped  off,  and  the  sample  carefully  weighed. 

Concrete  Brick.  —  Many  machines  are  now  on  the  market  for 
manufacturing  concrete  brick  to  be  used  in  place  of  the  ordi- 
nary clay  brick.  The  usual  size  is  about  2J  X  3|  X  8J  inches,  or 
about  the  same  as  clay  brick.  The  methods  of  manufacture 
and  treatment  are  much  the  same  as  in  the  case  of  concrete 
blocks,  though  the  compacting  is  usually  done  by  pressure  and 
no  cores  are  used.     Figure  60  shows  a  small  brick  machine. 


164    CONCRETE  CONSTRUCTION  FOR  RURAL  COMMUNITIES 

Some  of  the  advantages  claimed  for  these  brick  as  compared 
with  clay  brick  are: 


Fig.  60.  —  A  Concrete  Brick  Machine. 

1.  They  are  true  and  uniform  in  size,  thus  comparing  favorably 
with  pressed-clay  bricks. 

2.  They  are  cheaper  than  pressed-clay  brick  and  are  even  cheaper 
than  common  brick  in  many  localities. 

3.  They  are  more  durable. 

4.  They  are  more  pleasing  in  appearance  than  common  brick. 

5.  They  are  laid  better  and  more  cheaply  on  account  of  their 
uniformity  in  size. 

6.  They  can  easily  be  made  of  any  color  or  design. 

7.  They  absorb  less  moisture. 

8.  They  resist  frost  better. 

Building  Trim.  —  Window  and  door  sills  and  lintels  of  con- 
crete are  suitable  for  use  with  brick,  stone,  or  concrete  buildings. 
They  are  very  cheaply  and  easily  made,  and  are  satisfactory 
in  use.  Smooth-faced  blocks  are  preferable  for  this  use,  and 
these  may  readily  be  made  on  the  site  of  the  work,  or  even  in 
place  in  the  wall,  simple  board  forms  being  used  for  the  pur- 
pose. Sills  and  caps  should  be  reinforced  with  steel  rods,  these 
being  placed  near  the  upper  surface  for  the  sills  and  near  the 
lower  surface  for  the  caps.  Reinforcing  to  the  amount  of  about 
1  per  cent  of  the  cross-section  of  the  block  should  be  used. 
For,  example,  suppose  a  concrete  window  cap  8"  thick  and  12" 
high  is  to  be  built.     The  amount  of  reinforcing  to  be  used  is 


CASTING  IN  MOLDS 


165 


about  .01  X  8  X  12  =  0.96    sq.   inches.     Two  f-inch  round  rods 
placed  1|  inches  above  the  bottom  would  answer. 


Fig.  61.  —  A  Concrete  Block  House  with  Concrete  Trim. 

Many  forms  of  concrete  ornaments  are  made  for  use  in  build- 
ings and  elsewhere,  including  columns  with  their  bases  and  capi- 
tals, balusters,  railings,  cornices,  lattice  work,  vases,  fountains, 
and  statuary.  Molds  for  ornamental  casting  may  be  made 
from  wood,  cast  iron,  sand,  glue,  plaster  of  Paris,  or  any  other 
substance  which  will  hold  its 
shape  and  resist  the  pressure 
of  the  semifluid  concrete.  In 
much  of  the  higher  grade  of 
ornamental  work,  white  Port- 
land cement  and  white  sand  or 
marble  dust  are  used. 

Drain  Tile.  —  The  concrete 
pipe  industry  has  developed  to 
large  proportions  in  this  coun- 
try. The  sizes  used  run  from 
a  few  inches  to  many  feet  in 
diameter.     Pipes  larger  than  about  three  feet  in  diameter  are 


Fig.  62.  —  Hand  Mold  for  Making  Con- 
crete Tile  With  Bell  and  Spigot  Ends. 


166    CONCRETE  CONSTRUCTION  FOR  RURAL  COMMUNITIES 

usually  cast  in  place,  while  the  smaller  sizes  are  cast  separately  in 
units  similar  to  the  clay  tile  in  common  use.  All  but  the  smaller 
sizes  of  tile  are  usually  reinforced,  the  saving  due  to  the  decreased 
thickness  of  walls  being  greater  than  the  cost  of  the  steel. 


1 

F     "^--^  ^ 

If. 

■tes^^T ''****iti^'*  ^/m^i 

Fig.  63.  —  Power  Concrete  Tile  Machine. 

Among  the  advantages  of  concrete  pipe  over  clay  pipe  is  the 
fact  that  it  can  be  manufactured  almost  anywhere  and  the 
freight  charges  saved,  so  that  it  is  cheaper,  as  well  as  stronger 
and  truer,  than  clay  pipe.  Some  concrete  pipe  appears  to  have 
been  injured  by  the  alkali  in  the  soils  of  some  western  states, 
but  it  has  been  demonstrated  that  if  the  pipe  is  made  imper- 
vious to  water,  the  trouble  from  this  source  is  greatly  reduced. 


CASTING  IN  MOLDS 


167 


Very  simple  forms  may  be  used  for  the  manufacture  of  tile, 
consisting  essentially  of  outer  and  inner  collapsible  cylinders  be- 
tween which  the  concrete  is  rammed.  A  semi-dry  mixture  like 
that  used  for  concrete  blocks  is  used,  so  that  immediately  after 
the  compacting,  the  mold  may  be  removed  and  used  again. 


Fig.  64.  —  Curing  Yard  for  Concrete  Tile. 


When  large  quantities  of  tile  are  desired,  power-driven  ma- 
chines are  used,  in  which  the  operations  are  automatic,  all 
that  is  necessary  being  to  supply  the  machine  with  concrete  and 
to  remove  the  finished  tile. 

The  same  care  should  be  taken  in  the  curing  of  tile  as  of 
concrete  blocks,  keeping  it  for  a  week  in  the  curing  shed,  where 
it  is  protected  from  the  sun  and  wind,  and  moistening  it  by 
sprinkling.     It  should  not  be  laid  till  it  is  a  month  or  more  old. 

The  following  data  on  concrete  tile' are  given  by  the  Besser 
Manufacturing  Company.      (See  table  on  the  following  page.) 

Reinforced  Concrete  Pipe.  —  Several  systems  of  separately 
cast  reinforced  concrete  pipe  have  been  developed  for  use  where 


168    CONCRETE  CONSTRUCTION  FOR  RURAL  COMMUNITIES 

Table  X 
Concrete  Tile  Data  and  Costs 


"8 

u 

0>    TO 

"3 .2 

•S.s 
II 

73 

1 

E 

h 
11 

2 
o 

a 

i§ 

6 

1- 

S3  fl 

+»  eS 
in 

O  O 

•"2 

IS 

O  O 

|? 

111 

Average  capacity 
per  day  with  3  men 
to  do  all  the  work, 
2-foot  lengths. 

So 

a   ■ 

li 

1 

o 

&£ 

<D  ~ 
ft +5 

02    2 

8  « 

0*0 

it 

h 
! 

6 

u 

10 

$0.03 

60 

$0.02 

90 

$0.06 

$0.11 

$0.06 

$0.15 

8 

4 

6£ 

.06 

45 

.02 

80 

.07 

.15 

.08 

.20 

10 

it 

5 

.09 

32 

.03 

70 

.08 

.20 

.10 

.30 

12 

if 

3 

.12 

25 

.04 

65 

.09 

.25 

.13 

.36 

15 

if 

2 

.18 

19 

.05 

57 

.10 

.33 

.17 

.55 

18 

if 

It9* 

.21 

16 

.06 

50 

.11 

.38 

.19 

.80 

20 

2 

li 

.24 

14 

.07 

45 

.12 

.43 

.22 

1.00 

24 

2f 

ItV 

.34 

9 

.11 

40 

.14 

.59 

.30 

1.50 

30 

2| 

3 
4 

.45 

7 

.14 

35 

.16 

.75 

.38 

2.00 

36 

3 

2 
3 

.60 

6 

.18 

30 

.18 

.96 

.48 

2.50 

42 

3^ 

1 
2 

.75 

5 

.20 

17 

.35 

1.30 

.65 

48 

4 

1 
3 

1.15 

3^ 

.30 

10 

.65 

2.10 

1.05 

the  loads  are  too  heavy,  or  the  diameters  too  large,  for  plain 
concrete  to  be  used  economically.     In  these  the  reinforcement 


Fig.  65.  —  Reinforced  Concrete  Pipe,  Meriwether  System. 

is  allowed  to  project  from  both  ends  of  the  pipe,  so  that  when 
the  latter  is  laid  in  the  trench,  the  reinforcement  of  the  abutting 


CASTING  IN  MOLDS 


169 


ends  overlaps.  The  joint  is  then  slushed  up  with  a  rich  mortar, 
which  seals  it  and  ties  the  ends  together.  The  pipe,  when  laid, 
thus  becomes  practically  one 
continuous  piece.  Woven  wire 
or  expanded  metal  fabric  is 
usually  used  for  reinforcing. 

Concrete  F e n c e  Posts. — 
Concrete  is  being  used  to  a 
considerable  extent  to  replace 
wood  for  fence  posts,  hitching 
posts,  and  power  transmission 
line  poles.  When  properly 
made  these  are  satisfactory, 
though  some  complaint  has 
been  made  against  them  on 
account  of  the  failure  of  poorly 
made  posts.  For  best  results, 
the  posts  must  be  made  of 
the  proper  size,  from  good  con- 
crete and  with  sufficient  rein- 
forcing. Well-made  concrete 
posts  are  permanent,  and  will 
justify  a  somewhat  greater  ex- 
pense than  temporary  wooden 
or  metal  posts,  though  in  cer- 
tain places  concrete  posts  can 
be  made  as  cheaply  as  first- 
class  wooden  posts  can  be 
obtained. 

Molds  for  fence  posts  are 
made  of  cast-iron,  galvanized 
sheet  steel,  or  wood.  Several 
patented  forms  made  of  cast 
iron  or  sheet  steel  are  on  the 
market,  and,  if  one  is  going 
into  the  business  of  manufac- 


Fig.  66.  —  Power  Transmission  Line 
Pole  of  Reinforced  Concrete. 


turing  posts  on  a  large  scale,  the  purchase  of  such  forms  is  well 
worth  while.    Wooden  forms  can  be  made  very  cheaply,  however, 


170    CONCRETE  CONSTRUCTION  FOR  RURAL  COMMUNITIES 

and  will  answer  every  purpose  for  the  farmer  who  wishes  to  make 
only  his  own  posts  and  a  few  for  his  neighbors. 

The  sizes  for  posts  will  depend  somewhat  on  the  purpose  for 
which  they  are  to  be  used.  The  shapes  commonly  made  are 
rectangular,  triangular,  or  combined  rectangular  and  half-round. 
The  posts  usually  taper  from  the  bottom  to  the  top.  This  is 
as  it  should  be,  for  the  place  at  which  there  is  the  greatest 
tendency  to  break  off  is  at  or  near  the  ground.  Rectangular 
fence  posts  for  line  use  may  be  made  five  inches  by  six  inches 
at  the  butt  and  three  inches  by  four  inches  at  the  top.  For 
ordinary  use  they  may  be  made  about  seven  feet  long. 

A  form  which  can  readily  be  made  by  anyone  is  shown  in 
Fig.  67.    This  will  accommodate  six  posts.      Before  the  forms 


Fig.  67.  —  Wooden  Forms  for  Concrete  Fence  Posts. 


are  used,  they  should  be  given  a  coating  of  crude  oil  or  soft 
soap  to  prevent  adhesion  of  the  concrete,  and  this  coating  should 
be  repeated  as  often  as  necessary  during  the  use  of  the  molds. 
The  broken  stone  or  gravel  should  be  of  small  size,  not 
larger  than  |  to  f  inch.  A  1:2:4  mixture  will  usually  be  satis- 
factory.' The  concrete  should  be  mixed  to  a  mushy  or  semi- 
liquid  consistency,  so  that  it  can  be  compacted  by  joggling  it 
with  a  shovel.  The  tamping  which  would  be  necessary  with  a 
dryer  mix  would  be  likely  to  spring  the  forms  and  displace  the 
reinforcing. 


CASTING  IN  MOLDS  171 

The  reinforcement  should  consist  of  steel  rods  about  \  inch 
in  diameter.  One  of  these  rods  should  be  placed  near  each 
corner,  about  \  inch  from  each  side. 

A  high  carbon  steel  may  well  be  used  for  reinforcing  fence 
posts,  and  that  made  by  re-rolling  rails  should  be  entirely  suit- 
able. This  can  be  had  from  steel  supply  houses  very  cheaply 
and  will  make  stronger  posts  at  less  cost  than  the  softer  steel. 


Fig.  68.  —  Curing  Concrete  Fence  Posts. 

Many  posts  have  been  made  with  small  wires,  either  barbed 
or  smooth,  as  reinforcement,  and  a  good  deal  of  the  trouble  with 
broken  posts  can  be  traced  to  this  cause.  Wire  is  all  right  for 
reinforcing  posts,  provided  enough  of  it  is  used.  In  the  judg- 
ment of  the  author,  nothing  smaller  than  No.  6  wire  or  two 
strands  of  No.  10  wire  twisted  together  should  be  used.  If 
the  wire  is  bought  cut  to  length  and  bundled  up  straight,  like 
baling  wire,  it  will  be  found  much  more  convenient  to  use  than 
when  it  is  in  coils.  Plain  wire  should  be  used,  as  the  galvanized 
is  more  expensive  and  no  better. 

The  directions  given  for  curing  blocks  apply  equally  to  fence 
posts.  The  posts  should  not  be  used  for  about  a  month  after 
pouring,  and  care  should  be  taken  in  handling  them  while  they 
are  green,  not  to  subject  them  to  shocks  or  to  bending  stresses. 


172    CONCRETE  CONSTRUCTION  FOR   RURAL   COMMUNITIES 

They  should  be  placed  on  end  after  they  are  removed  from  the 
molds. 

Corner  and  Gate  Posts.  —  Corner  and  gate  posts  must  be 
made  considerably  heavier  than  line  posts,  as  the  pull  of  the 
fence  wire  comes  directly  on  them.  They  should  be  well  braced 
by  diagonal  reinforced  concrete  braces  running  downward  at  an 
angle  of  about  30  degrees  from  a  point  about  three-fourths  the 
height  of  the  fence  to  a  secure  footing  in  the  ground.  (See 
Fig.  69.)     The  upper  end  of  the  braces  may  be  made  beveled 


Fig.  69.  —  Concrete  Fence  Posts  and  Braces  at  Corner. 

to  fit  in  a  recess  or  against  a  bracket  cast  on  the  post.  The 
lower  end  of  the  brace  may  be  held  in  place  by  means  of  a 
small  mass  of  concrete  placed  when  the  post  is  set.  Diagonal 
wire  ties  may  also  be  used  on  the  post  next  to  the  corner  or 
gate  post.  Braced  posts  should  also  be  placed  at  intervals 
along  the  line  of  the  fence. 

Corner  and  gate  posts  are  usually  made  square  in  section  and 
without  taper.  The  sizes  commonly  used  are  8  by  8  or  10  by 
10  inches.  They  should  be  reinforced  by  half-inch  rods,  one  in 
each  corner,  about  three-quarters  of  an  inch  from  each  side. 
They  are  usually  cast  in  place,  in  wooden  or  metal  forms,  as 
they  are  too  heavy  to  be  moved  easily. 


CASTING  IN  MOLDS 


173 


Fastening  Fence  to  Posts.  —  Various  methods  have  been  de- 
vised for  attaching  the  fence  to  concrete  posts.  Probably  the 
most  satisfactory  method  is  to  pass  a  tie  wire  around  the  post 
on  the  side  opposite  the  fence  and  then  to  twist  its  ends  tightly 


Fig.  70.  —  Forms  for  Corner  Post  and  Braces 


about  the  wire  of  the  fence.  Two  methods  commonly  used  are 
shown  in  Fig.  71.  A  piece  of  strap  iron  bent  and  cut  as 
shown  in  the  figure  is  useful  in  making  these  twists.  In  either 
case  the  tie  wires  should  be  twisted  up  tightly  to  prevent  their 
slipping  along  the  post.  Grooving  the  face  of  the  post  when 
it  is  poured  will  assist  in  holding  the  wires  in  place.  The  wider 
sides  of  rectangular  posts  should  be  placed  at  right  angles  to  the 
line  of  the  fence,  as  they  are  stronger  when  placed  this  way. 

Cost  of  Posts.  —  The  cost  of  concrete  fence  posts  varies 
considerably  under  different  circumstances,  but  the  following 
estimate  will  serve  as  a  guide  in  computing  the  cost  in  any 
given  case.      (See  table  on  the  following  page.) 


174    CONCRETE  CONSTRUCTION  FOR  RURAL  COMMUNITIES 


/ 


/ 


Fig.  71.  —  Methods  of  Attaching  Wire  to  Concrete  Fence  Posts. 


Cost  of  Concrete  Fence  Posts 

(3"  x  4"  at  top  and  5"  x  6"  at  butt,  7  ft.  long,  of  1:  2:  4  gravel  concrete,  with 
four  \"  rods  6  ft.  10  in.  long) 

For  100  posts  there  will  be  required, 

3.1  yds.  gravel @  $1.25  $3.88 

1.6  yds.  sand     @      .75  1.20 

21.3  sacks  cement        @      .40  8.52 

456     lbs.  steel  rods @      .02^  11.40 

30  hours  labor      @      .20  6.00 

Cost  of  100  posts $31.00 

Cost  of  each  post 31  cents 

The  above  does  not  include  the  cost  of  forms,  as  this  may  be 
distributed  over  many  posts.  Where  the  posts  are  made  during 
spare  time,  and  scrap  steel  is  used  for  reinforcement,  the  cost 
may  be  considerably  reduced. 


PART  V 
TYPICAL  APPLICATIONS  OF  CONCRETE 


CHAPTER  XIII 
SIDEWALKS,    FLOORS,  AND    ROADS 

The  principles  brought  out  in  the  preceding  chapters  apply 
to  the  use  of  concrete  for  all  purposes.  The  essential  factors 
for  success  are  careful  selection  of  materials,  use  of  suitable 
proportions,  proper  methods  of  mixing,  handling,  and  placing 
the  concrete,  and  the  preparation  of  suitable  forms.  These 
matters  have  all  been  discussed.  In  the  remaining  chapters, 
more  detailed  directions  will  be  given  for  the  construction  of  a 
few  typical  structures  selected  from  those  widely  used  in  rural 
communities. 

SIDEWALKS 

Preparation  of  Subgrade.  —  The  preparation  of  a  suitable 
foundation  for  a  sidewalk  is  very  important.  If  the  walk  is  to 
be  laid  on  a  new  fill,  the  latter  should  be  placed  in  layers  not 
thicker  than  six  inches,  each  of  which  should  be  thoroughly 
rolled  or  tamped  to  prevent  future  settlement.  A  liberal  use 
of  water  on  the  fill  will  often  help  in  compacting  it. 

If  the  soil  is  stiff  and  clayey,  the  foundation  should  be  well 
drained.  This  can  be  accomplished  by  placing  the  walk  on  a 
sub-base  consisting  of  a  layer  of  cinders,  gravel,  or  broken  stone 
from  four  to  eight  inches  in  thickness,  depending  on  the  char- 
acter of  the  soil,  the  climate,  and  the  width  of  the  walk.  This 
material  should  be  thoroughly  compacted  by  tamping  or  rolling. 
If  the  soil  is  very  stiff,  drains  of  porous  material  or  of  tile  should 
be  placed  at  the  lowest  points  to  carry  off  any  water  which  may 
accumulate  in  this  sub-base. 

In  mild  climates  where  the  soil  is  sandy,  the  sub-base  may 
be  omitted. 

Proportions.  —  Walks  are  generally  made  of  two  courses,  a 
thick  base  of  concrete  and  a  thin  wearing  surface  of  rich  mor- 

177 


178     CONCRETE  CONSTRUCTION  FOR  RURAL  COMMUNITIES 

tar.  For  widths  of  four  feet  or  more,  the  base  should  be 
about  four  inches  thick,  and  the  top  coat  about  three-fourths 
inches.  For  narrow  walks  the  base  may  be  made  two  and  one- 
half  to  three  inches  thick,  and  the  top  coat  about  one-half 
inch. 

The  proportions  for  the  base  should  be  about  1 :  2§ :  5  and 
for  the  top  about  1:1  J.  With  extra  good  materials  1:3:6 
may  be  used  for  the  base  and  1:2  for  the  top.  The  coarse 
aggregate  should  not  be  larger  than  1J  inch. 

On  account  of  the  extra  labor  required  in  providing  contrac- 
tion joints  when  broken  stone  or  gravel  is  used,  it  is  a  common 
practice  in  many  places  to  omit  the  coarse  aggregate,  or  some- 
times to  use  a  very  fine  gravel  or  broken  stone.  In  such  cases 
proportions  of  1:4  or  1:3:4  may y  be  successfully  used  for  the 
base.  With  a  good  bank-run  gravel  the  proportion  of  1:5  is 
used. 

Single  course  walks  are  coming  into  use  to  some  extent.  In 
these  the  whole  depth  of  the  walk  is  made  of  a  rich  mixture 
with  an  excess  of  mortar,  such  as  1:2:3}.  By  tamping,  the 
coarse  aggregate  is  forced  down,  leaving  only  mortar  on  the 
surface.  This  is  leveled  down  and  finished  in  the  usual  way. 
One  of  the  arguments  advanced  for  this  method  is  that  it  elimi- 
nates any  danger  of  separation  of  the  top  coat  from  the  base. 

Where  driveways  cross  walks,  the  thickness  of  the  base 
should  be  increased  to  six  inches  and  of  the  top  to  one  inch  or 
more. 

Forms.  —  For  straight  walks  forms  are  usually  made  of 
wooden  two-by-fours  laid  on  edge  and  held  in  place  by  stakes 
driven  on  the  outside.  The  forms  may  be  lightly  nailed  to 
these  stakes  to  hold  them  at  the  proper  height.  For  making 
curves,  one-by-fours  or  one-half  by  four  inch  pieces  are  used. 
The  forms  on  one  side  should  be  placed  a  little  lower  than  on 
the  other  to  insure  that  no  water  will  stand  on  the  walk  during 
a  rain.  A  drop  of  one-fourth  inch  for  each  foot  of  width  will 
be  sufficient. 

If  coarse  aggregate  is  used  in  the  concrete,  then  cross  forms 
will  be  needed  to  divide  the  walk  into  blocks.  These  may  be 
made  of  two-by-fours  so  placed  that  alternate  blocks  can  be 


SIDEWALKS,   FLOORS,  AND  ROADS 


179 


poured.  After  the  base  of  these  blocks  has  been  placed,  the 
cross  forms  are  removed  and  the  intermediate  spaces  are  filled. 
Marks  should  be  made  on  the  side  forms  at  each  joint  of  the 
base,  so  that  the  grooves  in  the  top  coat  can  be  located  accu- 
rately over  these  joints.  Failure  to  do  this  has  caused  many 
unsightly  cracks.  Another  method,  which  has  much  to  recom- 
mend it,  is  to  use  sheet  steel  dividing  plates  about  one-eighth 
inch  thick  between  adjacent  blocks.  These  are  made  with 
hooks  on  the  ends,  which  hold  the  side  forms  in  place.  When 
the  surface  is  ready  to  be  finished,  the  dividing  plates  are  re- 
moved and  a  groover  is  run  over  the  joints. 


Fig.  72  —  Sheet  Steel  Sidewalk  Forms. 

Sheet-steel  side  forms  are  being  used  to  some  extent  and  are 
proving  very  satisfactory.     Figure  72  shows  one  of  these  forms. 

Mixing  and  Placing  the  Concrete.  —  A  concrete  of  dry  con- 
sistency is  often  used  for  the  base  of  sidewalks.  It  is  better  to 
use  a  mushy  concrete,  as  less  labor  is  required  in  compacting 
and  better  results  are  obtained.  A  straight-edged  board,  notched 
over  the  side  forms  to  leave  a  proper  depth  for  the  top  coat, 
may  be  used  to  level  off  the  base.  The  top  coat  should  be 
mixed  wet,  but  not  so  wet  as  to  give  any  surplus  water  when 
it  is  leveled  off.  It  is  a  common  fault  to  mix  the  top  coat  too 
wet. 

The  top  should  be  placed  on  the  base  as  soon  as  possible 
after  the  latter  is  poured.     If  it  is  not  placed  within  half  an 


180    CONCRETE  CONSTRUCTION  FOR  RURAL  COMMUNITIES 

hour,  an  imperfect  bond  between  the  two  layers  is  likely  to  be 
obtained.  This  is  one  of  the  most  common  causes  of  the  fail- 
ure of  concrete  sidewalks. 

Provision  must  be  made  for  contraction  joints,  or  the  walk 
will  crack  irregularly.  If  alternate  blocks  are  first  placed  and 
then  the  intermediate  sections  are  filled  in,  a  sheet  of  tar  paper 
between  the  blocks  will  maintain  a  good  joint.  If  steel  parting 
plates  are  used,  these  will  of  course  provide  both  contraction 
and  expansion  joints.  When  the  coarse  aggregate  is  omitted, 
it  is  a  common  practice  to  make  these  joints  by  cutting 
through  both  top  and  base  with  a  trowel,  after  the  concrete  has 
stiffened  up  sufficiently.  If  this  is  done,  great  care  must  be 
taken  to  see  that  the  base  is  cut  entirely  through,  as  otherwise 
the  cracks  may  not  follow  the  groove  in  the  surface.  The 
size  of  blocks  should  be  limited  to  about  six  feet  wide  by 
five  feet  long,  for  walks  4|  inches  thick.  If  the  walks  are  thin- 
ner, the  maximum  dimension  of  the  blocks  may  be  made  about 
twelve  times  the  thickness. 

Expansion  joints  about  one-half  inch  wide  should  be  left  entirely 
through  the  walk  at  intervals  of  fifty  feet  or  less  in  straight  runs, 
and  on  each  side  of  turns,  unless  a  small  space  is  allowed  between 
each  block.  When  metal  parting  strips  are  used  between  blocks, 
and  are  left  in  place  until  the  top  coat  is  placed  and  finished,  no 
additional  provision  for  expansion  need  be  made. 

Surface  Finish.  —  The  top  of  the  wearing  coat  should  be 
struck  off  even  with  the  side  forms  with  a  straight  edge  imme- 
diately after  it  is  placed.  As  soon  as  the  free  water  has  dis- 
appeared, it  should  be  worked  over  with  a  wooden  float  to  take 
out  all  irregularities  and  to  compact  it.  This  will  leave  a  rough, 
pleasing  finish  which  will  not  be  slippery  in  wet  or  icy  weather. 
An  edging  tool  may  be  run  next  to  the  side  forms,  and  a  groover 
across  the  joints,  to  give  a  border  to  the  blocks  and  to  round 
the  edges  off.  The  wooden  float  can  easily  be  made  by  nailing 
a  cleat  for  a  handle  on  a  one-by-six  inch  board  about  fifteen 
inches  long.  The  edger  and  groover  can  be  bought  very  cheaply 
at  hardware  stores. 

If  it  is  desired  to  have  a  smoother  finish  than  the  above,  it 
can  be  obtained  by  means  of  a  steel  trowel,  manipulated  with  a 


SIDEWALKS,   FLOORS,  AND  ROADS  181 

circular  motion,  while  the  surface  is  still  rather  soft.  This  will 
give  a  finish  which  is  not  so  smooth  as  to  be  slippery,  nor  yet 
so  rough  as  to  be  difficult  to  keep  clean. 


Fig.  73.  —  Concrete  Sidewalk  with  Floated  Finish. 

The  smooth,  glassy  finish  obtained  by  troweling  the  surface 
after  it  has  become  almost  hard  is  objectionable  for  the  reasons 
that  it  is  slippery,  that  it  gives  a  disagreeably  intense  reflection 
of  the  sun  in  hot  weather,  and  that  working  the  concrete  after 
it  has  begun  to  set  is  injurious  to  it.  Either  the  wooden- 
floated  or  the  steel-floated  finish  is  much  to  be  preferred. 

Curing.  —  In  very  dry,  hot  weather  the  surface  of  the  walk 
should  be  covered  with  sand,  earth,  or  other  material,  as  soon 
as  the  concrete  has  hardened  sufficiently,  and  this  covering 
should  be  kept  wet  for  about  four  days.  In  cool  or  cloudy 
weather  the  covering  may  be  omitted,  but  the  surface  should 
be  kept  moist  for  a  few  days.  No  traffic  should  be  allowed  on 
the  walk  for  at  least  forty-eight  hours,  or  longer  in  cool  weather. 

If  a  rainstorm  should  come  up  before  the  walk  is  hardened 
sufficiently  to  resist  pitting,  it  can  be  protected  with  an  inch 
of  sand. 

Cost  of  Sidewalks.  —  The  cost  of  materials  and  labor  for 
concrete  sidewalks,  including  the  preparation  of  the  sub-base, 
will  usually  range  from  10  to  15  cents  per  square  foot,  depend- 
ing on  local  conditions.  Walks  on  the  grounds  of  the  Kansas 
State  Agricultural  College  cost  about  8  cents  per  square  foot, 
not  including  the  preparation  of  the  sub-base. 


182    CONCRETE  CONSTRUCTION  FOR  RURAL  COMMUNITIES 

FLOORS 

Use  of  Concrete  for  Floors.  —  Concrete  is  being  used  to  a 
large  extent  for  floors  for  all  purposes,  especially  those  placed 
directly  on  the  ground.  It  is  used  for  the  first  or  basement 
floor  of  warehouses,  factories,  dwellings,  garages,  barns,  dairies, 
and  granaries,  for  feeding  floors,  for  barnyard  floors,  and  where- 
ever  it  is  desired  to  have  a  cheap,  clean,  sanitary,  and  per- 
manent surface. 

The  methods  used  for  sidewalk  construction  will  generally 
apply  equally  well  for  floors. 

Cellar  Floors.  —  Cellar  floors  do  not  usually  require  any  po- 
rous foundation,  and  need  be  only  three  to  four  inches  thick 
unless  they  are  to  receive  hard  usage.  A  one  course  floor 
mixed  1 :  2\ :  4  may  be  used,  or  a  top  of  1:2  about  half  an  inch 
thick  may  be  placed  on  a  1 :  2J :  5  base.  On  account  of  the 
smaller  temperature  range,  blocks  may  be  a  littler  larger  than 
for  sidewalks,  say  about  six  or  eight  feet  square.  A  smooth 
troweled  finish  will  usually  be  preferred,  but  the  troweling 
should  be  done  before  the  surface  has  become  too  stiff. 

Barn  Floors.  —  Barn  floors  should  be  made  like  sidewalks, 
with  a  porous  sub-base  about  six  inches  thick,  while  the  floor 
itself  is  about  five  inches  thick  of  either  one  or  two  courses. 
If  two  courses  are  used,  the  mortar  top  should  be  about  one 
inch  thick.  Sufficient  slope  must  be  given  to  the  floor  to  carry 
the  liquids  to  the  drains.  A  semi-rough  finish  should  be  used, 
so  that  animals  will  not  slip  on  it. 

3-6- 

Feed  Trough  „     I   „ 

Rods  |  15  on  Centers 


Fig.  74.  —  Section  of  Concrete  to  Floor  for  Dairy. 

Figure  74  shows  a  section  of  a  concrete  dairy  floor  for  use 
with  iron  pipe  stanchions.  The  feed  trough  can  easily  be  made 
by  building  board  forms  of  the  proper  height  on  the  two  sides, 
and  scraping  out  the  interior  by  means  of  a  template  made  up 


SIDEWALKS,  FLOORS,  AND  ROADS 


183 


of  boards  cut  to  the  desired  shape  and  supported  by  the  forms 
on  each  side,  as  shown  in  Fig.  75.  Means  should  be  pro- 
vided to  drain  the  feed  trough  in  order  that  it  may  be  used 
for  watering  the  cattle  also. 


Template  of  1  In. Boards 


In.Board3 

2x4  In. Cleats 


Fig.  75.  —  Forms  and  Template  for  Feed 
Trough  of  Dairy  Floor. 

Feeding  Floors.  —  Feeding  floors  are  a  profitable  investment 
on  farms  where  much  live  stock  is  kept.    They  may  easily  save 


Fig.   76.  —  Interior  of  Dairy  Barn  with  Concrete  Floor  and  Iron  Pipe 

Stanchions. 


184    CONCRETE  CONSTRUCTION  FOR  RURAL  COMMUNITIES 

their  cost  in  one  year  in  the  saving  of  feed  and  fertilizer  and 
in  the  improved  condition  of  the  animals.  Concrete  is  by  far 
the  most  suitable  material  for  their  construction. 

Exactly  the  same  materials  and  methods  should  be  used  for 
making  feeding  floors  as  for  sidewalks.  A  wall  about  six  inches 
thick  and  eighteen  inches  deep  should  be  placed  under  the  outer 


Fig.  77.  —  Concrete  Feeding  Floor. 

edge  of  the  floor,  on  all  sides,  to  keep  hogs  from  undermin- 
ing it,  and  to  keep  rats  out.  The  floor  should  be  given  a 
slope  of  one-fourth  inch  per  foot  of  width,  to  insure  drainage.  A 
gutter  may  be  made  at  the  lower  edge  to  carry  away  the  water. 
If  desired,  the  gutter  may  be  connected  with  a  concrete  manure 
pit,  so  that  all  the  manure  washed  off  by  rains  will  be  saved. 

Feeding  troughs  for  hogs  may  be  made  of  concrete  as  a  part 
of  the  feeding  floors,  as  shown  in  Fig.  78. 

CONCRETE   ROADS 

Concrete  is  being  extensively  used  for  road  surfaces,  both  in  the 
cities  and  in  the  country,  and  its  use  for  these  purposes  will  un- 
doubtedly be  greatly  extended  in  the  future.  It  is  much  cheaper 
than  brick  in  first  cost,  and  in  upkeep  is  considerably  less  ex- 


SIDEWALKS,   FLOORS,   AND  ROADS 


185 


pensive  than  macadam.  On  account  of  the  severe  usage  to  which 
roads  are  subjected,  and  the  expense  of  their  construction,  a 
competent  engineer  should  always  be  employed  to  prepare  plans 


Fig.  78.  —  Feed  Trough  for  Hogs,  Built  as  Part  of  Feeding  Floor. 

and  specifications  and  to  supervise  their  construction.1  Many 
times  the  amount  paid  for  his  services  may  be  saved  in  the 
longer  life  and  lower  expense  for  repairs  of  the  road. 

Construction  of  Roads. — The  general  methods  used  in  road  con- 
struction are  the  same  as  for  sidewalks,  but,  on  account  of  the  more 
severe  use  to  which  roads  are  subjected,  somewhat  greater  care 
must  be  taken  in  the  selection  of  materials  and  in  the  prepara- 
tion of  the  sub-base,  and  richer  and  thicker  concrete  must  be  used. 

Good  drainage  must  be  provided  and  the  sub-grade  and 
porous  sub-base  should  be  thoroughly  rolled  with  a  five  or  ten 
ton  roller.  All  vegetable  or  other  perishable  matter  must  be 
entirely  removed.  The  top  of  the  sub-base  should  be  flat  for 
the  full  width  of  the  concrete. 

The  width  to  be  paved  will,  of  course,  vary  with  local  condi- 

1  Standard  specifications  for  concrete  highways  and  for  one-  and  two-course 
concrete  pavements  may  be  obtained  from  the  American  Concrete  Institute, 
Philadelphia,  Pa.,  the  Association  of  American  Portland  Cement  Manufactur- 
ers, Philadelphia,  Pa.,  or  the  Office  of  Public  Roads  and  Rural  Engineering, 
U.  S.  Dept.  of  Agriculture,  Washington,  D.  C. 


186    CONCRETE  CONSTRUCTION  FOR  RURAL  COMMUNITIES 


tions.  It  should  preferably  be  enough  to  enable  vehicles  to 
pass  each  other,  but  this  is  not  absolutely  necessary  if  the 
shoulders  are  well  graded  up  on  each  side  of  the  concrete.    The 


Not  more  thany^r 


^Arc  of  Circle 


Slope  1H to 


-Arc  of  Circle 


SECTION  ON  FILL 
ItolV 

Sloped  to  10- 


About  1 


Note-.-W    Denotes  WLat'h  of  Pavement 


Fig.  79.  —  Sections  of  Concrete  Roadway  Recommended  by  American 
Concrete  Institute.    . 

section  of  the  road  should  be  an  arc  of  a  circle,  with  the  middle 
higher  than  the  edges  by  about  ywu  °f  the  width.  Thus  in  a 
road  sixteen  feet  wide,  the  amount  of  crown  would  be  about 
T\nr  X  12  =  1.9  inches.     The  concrete  should  be  not  less  than  six 


Fig.  80.  —  Steel  Plates  for  Protecting  Joints  in  Concrete  Roadway. 

inches  thick  at  the  edges,  and  at  the  middle  it  will  be  thicker  than 
this  by  the  amount  of  the  crown.  Figure  79  shows  the  sections 
of  roadway  recommended  by  the  American  Concrete  Institute. 


SIDEWALKS,   FLOORS,  AND  ROADS 


187 


Traverse  joints  one-fourth  inch  wide  should  be  located  not 
more  than  thirty-six  feet  apart.  They  should  be  filled  with 
prepared  strips  of  fiber  and  bitumen  extending  the  full  thickness 
of  the  pavement.  Frequently,  soft  steel  plates  one-eighth  inch 
thick,  well  anchored  to  the  concrete,  are  used  on  each  side  of 
the  joints  to  keep  them  from  being  broken  down  by  the  traffic. 


Fig.  81.  —  Concrete  Roadway  on  Campus  of  the  Kansas  State  Agricultural 

College. 

The  proportions  for  single  course  roads  should  be  about 
1:2:3  or  1:2:4.  The  concrete  should  be  finished  with  a 
wooden  float,  or  with  a  wire  broom  if  a  rougher  surface  is 
desired.  It  should  be  protected  from  the  sun  by  two  inches  of 
sand  or  earth  which  is  kept  wet  for  at  least  ten  days,  if  the 
temperature  is  higher  than  50  degrees  Fahrenheit.  Under  the 
most  favorable  conditions  the  road  should  not  be  opened  to 
traffic  earlier  than  fourteen  days  after  being  poured,  and  in 
most  cases  three  weeks  or  a  month  should  be  allowed. 

Cost  of  Roads.  —  The  cost  will  usually  range  from  about  $1  to 
$1.25  per  sq.  yd.,  with  average  cost  of  materials  and  under  average 
conditions,  for  either  one  or  two  course  roads.  For  a  road  10 
feet  wide,  at  $1  per  square  yard,  the  cost  per  mile  is  $5867. 


CHAPTER  XIV 

TANKS,   CISTERNS,  AND  SILOS 

TANKS 

Use  of  Concrete  for  Water  Tanks.  —  Concrete  possesses  many 
advantages  over  other  materials  used  for  water  tanks,  It  is 
cheap  and  easily  built  into  any  desired  form ;  it  will  not  decay  or 


Fig.  82.  —  Small  Reinforced  Concrete  Tank. 

rust;  it  is  substantial,  and  will  not  shrink  and  collapse  or  become 
leaky  if  left  empty;   and  it  is  sanitary  and  vermin  proof. 

The  two  shapes  most  used  for  tanks  are  rectangular  and 
round.  The  former  has  the  advantage  that  the  forms  are  easier 
to  build,  while  the  latter  requires  less  reinforcing  and  less  con- 
crete for  the  same  capacity.  Large  tanks  are  therefore  usually 
made  circular  in  shape,  while  small  ones  are  frequently  made 
rectangular. 

188 


TANKS,  CISTERNS,  AND  SILOS  189 

Construction  of  Tanks.  —  After  the  site  of  a  tank  is  selected, 
the  ground  should  be  leveled  off  and  excavated  to  a  depth  of 
six  or  eight  inches,  or  until  a  solid  foundation  is  obtained. 
Any  soft  spongy  spots  should  be  removed  and  refilled  with  good 
material.  If  the  condition  of  the  ground  requires  it,  a  fill 
should  be  made  of  about  six  inches  of  cinders  or  gravel  well 
tamped.  In  large  tanks  or  on  poor  ground  it  may  be  desirable 
to  construct  a  foundation  of  lean  concrete. 


Fig.  83.  -—  Forms  for  Rectangular  Tank. 

The  forms  for  either  rectangular  or  round  tanks  may  be  built  by 
the  methods  described  in  Chapter  V.  If  several  round  tanks  of  the 
same  size  are  to  be  made,  it  will  be  found  convenient  to  use  sheet- 
metal  forms,  which  can  be  cheaply  made  up  at  any  tin  shop. 

Proportions  of  1:2:4  or  1:2:3J  should  ordinarily  be  used. 
If  gravel  is  difficult  to  obtain,  1 :  2\  will  answer.  The  addition 
of  hydrated  lime,  up  to  about  10  per  cent  of  the  amount  of  the 
cement,  is  recommended.  Enough  water  should  be  used  to  give 
a  wet  consistency. 

The  method  of  construction  differs  somewhat  with  the  size 


190    CONCRETE  CONSTRUCTION  FOR  RURAL  COMMUNITIES 

of  the  tank.  For  small  tanks,  after  the  outer  forms  are  in 
place  and  are  well  braced,  the  reinforcing  for  the  floor  and  sides 
should  be  placed  and  wired  in  position,  and  overflow  and  drain 
pipes  put  into  place.  The  inside  form  should  next  be  put  to- 
gether, being  wired  to  the  outer  form  and  cross-braced  to  the 
opposite  side.  It  may  be  supported  from  the  outer  form  by 
pieces  nailed  to  the  studs  and  carried  across  the  tank. 

If  the  floor  has  become  hard  before  the  sides  are  poured,  or 
if  the  sides  are  not  poured  in  a  continuous  run,  the  precautions 
discussed  in  Chapter  XI  should  be  observed,  to  prevent  leak- 
age at  the  joints  between  the  old  and  the  new  work.  It  is  very 
desirable  that  tanks  be  poured  continuously  if  possible. 

Table  XI 
Dimensions  and  Reinforcement  for  Circular  Water  Tanks 


Inside 

Depth 

of  tank 

ft. 

Thickness 

of  walls, 

in. 

Capacity 
gal. 

Horizontal  rods  in  wall 

Vertical  rods 

diam. 

of  tank, 

ft. 

Diameter, 
,        in. 

Spacing 

at  bottom 

in. 

Spacing 

at  top, 

in. 

Diameter, 
in. 

Spacing, 
in. 

6 

6 

9 

9 

9 

12 

12 

12 

12 

3 
6 
3 
-6 
9 
3 
6 
12 
18 

6 
6 
6 
6 
6 
6 
6 
8 
8 

635 
1270 
1430 
2860 
4290 
2540 
5070 
10150 
15200 

l 

4 
1 

4 

1 
4 
1 
4 
3 
8 
1 
4 
3 
8 
3 
8 
1 
2 

12 

7 
9 
5 
7 
7 
8 
4 
5 

15 
15 
15 
15 
15 
15 
15 
20 
20 

l 

4 

4 
1 
4 
1 
4 
3 
"8 
A 
4 
1 
4 
1 
8 

8 

36 
30 
36 
30 
36 
36 
30 
36 
30 

Note.  —  Increase  spacing  of  horizontal  rods  gradually  from  bottom  to 
top  of  wall.    Place  rods  about  \\  inches  from  outer  face  of  wall. 

For  reinforcing  floor  when  tank  rests  on  firm  soil,  use  same  size  and  spacing 
as  for  horizontal  rods  at  top  of  wall.  Run  floor  rods  both  ways,  placing  them 
about  one  inch  above  bottom  of  concrete,  and  extending  ends  12  to  15  inches 
up  into  the  walls.    Make  floors  of  same  thickness  as  walls. 

The  forms  may  be  removed  in  two  or  three  days,  and  the 
walls,  both  inside  and  outside,  should  then  be  given  two  brush 
coats  of  neat  cement,  or  of  equal  parts  of  cement  and  fine 
sand,  mixed  with  water  to  a  creamy  consistency.  This  will 
help  to  fill  up  the  pores  and  make  the  work  water-tight. 


TANKS,  CISTERNS,  AND  SILOS  191 

Table  XII 

Dimensions  and  Reinforcement  for  Square  or  Rectangular 
Water  Tanks 


£ 

<n 

Horizontal  rods  in  walls.1 

Vertical  rods 

-ti 

A 

a 

3 

c4 

03 

f 

J- 

.9 

01 

ts.S 

m 

2d 

pi 

«e  » °  j 

^^  o  o>  d 
a>  os  «  — 

a"°4 

•  5 

.s 

•a 

a 

1 

o5 

E 

5 

■a  o 

IS 

a  o 

CO.Q 

oo -2 

Q   OBiO 

s 

cS 

S 

§ 

a 
aS 

4 

3 

360 

6 

i" 

7 

18 

1 

3  If 
8 

27 

6 

3 

800 

6 

3  " 

5 

18 

1 

3// 

8 

27 

8 

3 

1435 

6 

1  n 

2 

5 

18 

1 

3* 
1 

27 

12 

3 

3225 

8 

5  // 
8 

6 

18 

n 

3  If 

8 

27 

16 

3 

5750 

9 

3// 

5 

18 

H 

3  If 
8 

27 

6 

6 

1600 

6 

5// 

7 

18 

l] 

3// 

8 

23 

12 

6 

6450 

10 

3.// 

5 

24 

H 

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37 

1  Note.  —  Increase  spacing  of  horizontal  rods  gradually  from  bottom  to 
top  of  walls.  Place  vertical  rods  inside  horizontal  rods,  wiring  the  latter  at 
exactly  the  given  distances  from  outer  faces  of  walls.  Hook  horizontal  bars 
together  at  corners,  or  make  them  continuous. 

For  reinforcing  of  floor,  when  tank  rests  on  firm  soil,  use  same  size  and 
spacing  as  for  vertical  rods.  Run  floor  rods  both  ways,  placing  them  one  to 
two  inches  above  to  bottom  of  concrete  and  extend  ends  up  to  make  vertical 
reinforcing  for  walls.  Make  floors  of  same  thickness  as  for  circular  tanks  of 
same  depth  with  diameter  equal  to  length  of  side. 

Reinforcing.  —  The  amount  of  reinforcing  required  in  the 
walls  of  circular  tanks  can  easily  be  computed  by  the  method 
given  in  Chapter  VIII.  The  sides  of  rectangular  tanks  may 
be  considered  as  slabs  on  edge,  supported  at  the  ends  and  loaded 
with  the  pressure  of  the  water.  The  reinforcing  of  the  bottom 
is  largely  a  matter  of  judgment.  Woven  wire  fabric  is  satis- 
factory for  small  tanks.  The  bottom  reinforcement  should  be 
extended  up  into  the  sides,  to  tie  these  parts  together.  Tables 
XI  and  XII  give  dimensions  and  data  for  reinforcing  various  sizes 
of  circular  and  square  or  rectangular  tanks.  Table  XII  applies 
equally  well  to  any  width  of  tank  not  greater  than  the  length. 

The  reinforcement  for  the  walls  should  be  placed  near  the 
outer  face.  For  rectangular  tanks  about  three-fourths  inch  to 
one  inch  in  the  clear  should  be  allowed.    For  circular  tanks  the 


192    CONCRETE  CONSTRUCTION  FOR  RURAL  COMMUNITIES 

exact  distance  is  not  important,  but  one  or  two  inches  is  satis- 
factory. Where  bars  are  spliced,  they  should  be  hooked  together 
and  wired  taut,  or  should  be  lapped  thirty  diameters  if  they 
are  deformed,  or  sixty  diameters  if  plain. 

Circular  tanks  are  frequently  built  by  plastering  on  metal 
lath,  much  as  plastered  concrete  silos  are  made.  If  the  tanks 
are  of  large  diameter  or  are  tall,  additional  reinforcement,  in 
the  form  of  bars,  will  be  required. 

CISTERNS 

Cisterns  are  usually  underground  and  may  be  made  either  round 
or  square  in  section.  If  square,  the  thickness  of  the  sides  should 
be  about  fV  of  the  length;  if  round  a  little  less  may  be  used. 
The  floor  should  usually  be  six  to  eight  inches  thick.  A  cistern 
eight  feet  square  and  eight  feet  deep  will  hold  about  120  barrels. 

If  the  earth  will  hold,  it  may  be  used  for  the  outer  form  and 
the  excavation  will  be  made  only  the  size  of  the  cistern  plus 
the  thickness  of  the  walls.  The  inner  form,  built  so  that  it 
may  be  readily  taken  to  pieces,  should  be  set  in  place  on  bricks 
or  stones  of  the  thickness  of  the  floor,  with  one-inch  wedges 
between  the  ends  of  the  studs  and  the  bricks. 

The  concrete  should  be  made  of  the  proportions  1:2:4, 
mixed  to  a  mushy  consistency.  The  floor  should  be  placed  and 
well  compacted,  after  which  the  side  forms  should  be  filled. 
Special  care  should  be  taken  to  work  the  concrete  around  the 
blocks  used  for  supporting  the  side  forms,  and  to  see  that  no 
dirt  from  the  wall  falls  into  the  concrete.  Connections  should 
be  inserted  at  the  proper  heights  for  a  pipe  to  the  house  pump, 
for  a  leader  from  the  downspout,  and  for  an  overflow.  The 
concrete  for  the  floor  and  walls  should,  if  possible,  be  placed 
in  a  continuous  run. 

A  reinforced  concrete  slab  should  be  placed  over  the  cistern. 
The  forms  for  this  can  be  supported  on  the  side  forms.  An 
opening  for  a  manhole  can  be  made  by  building  a  bottomless 
box  of  a  height  equal  to  the  thickness  of  the  slab,  2  feet 
square  at  the  top  and  18  inches  square  at  the  bottom.  This  is 
lightly  nailed  to  the  top  form  at  the  desired  place.  The  slab 
should  be  made  to  slope  away  from  the  manhole,  about  one- 


TANKS,  CISTERNS,  AND  SILOS  193 

fourth  inch  per  foot.  For  an  eight-foot  square  cistern,  the  top 
may  be  made  five  inches  thick,  reinforced  with  f-inch  round 
rods  spaced  one  foot  apart,  and  running  in  both  directions. 
The  steel  should  be  about  half  an  inch  in  the  clear  from  the 
bottom  of  the  slab.  One  additional  rod  should  be  placed  on  each 
side  of  the  manhole.      Figure  84  shows  such  a  cistern  cover. 


Fig.  84.  —  Concrete  Top  and  Manhole  Cover  for  Cistern. 

The  form  for  the  manhole  may  be  removed  in  a  few  hours, 
and  the  cover  cast  in  place.  To  make  the  latter  lighter,  two 
inches  of  sand  may  be  put  on  the  forms  before  placing  the 
concrete.  It  should  be  reinforced  by  three  rods  in  each 
direction,  placed  near  the  bottom.  A  ring  should  be  fastened 
in  the  cover  for  convenience  in  lifting  it. 

In  two  weeks,  the  manhole  cover  may  be  lifted,  and  a  hole 
sawed  through  the  forms  to  permit  access  to  the  cistern,  after 
which  the  forms  may  be  torn  apart  and  passed  out  through  the 
opening.  The  walls  should  then  be  given  two  coats  of  neat 
cement  mixed  with  water  to  a  creamy  consistency  and  applied 
with  a  brush. 

A  concrete  filter  may  be  made  for  the  cistern  in  much  the 
same  manner  that  the  cistern  itself  is  constructed. 

SILOS 

Requirements  of  a  Good  Silo.  —  In  order  that  silage  may 
keep  well,  the  walls  of  the  silo  must  be  air-  and  water-tight. 
Silage  will  mold  and  spoil  if  air  comes  into  contact  with  it, 
while  if  the  wall  is  not  water-tight  the  juices  will  leak  out  and 
be  lost,  and  the  silage  acids  which  penetrate  the  concrete  will 
tend  to  disintegrate  it.     The  inner  face  of  the  silo  should  be 


194    CONCRETE  CONSTRUCTION  FOR  RURAL  COMMUNITIES 

smooth,  so  that  air  pockets  will  not  be  formed  when  the  silage 
settles.  The  walls  should  not  conduct  heat  readily,  as  silage 
will  keep  better  if  it  is  not  subjected  to  extremes  of  temperature. 
Freezing  of  silage  in  winter  is  a  source  of  annoyance  and  loss. 


Fig.  85.  —  Note  the  Superior  Appearance  of  the  Plastered  Concrete  Silo.     It 
is  also  Permanent  and  there  is  no  Upkeep  Expense. 

Concrete  silos  when  properly  made  embody  the  desirable 
features  mentioned.  In  addition,  they  are  strong  and  rigid 
and  will  not  blow  down  in  strong  winds,  nor  collapse  from  dry- 
ing out  in  the  summer.  They  will  not  burn  nor  decay,  they 
are  rat-  and  vermin-proof,  they  will  keep  the  silage  as  well  as 
any  other  type,  and  if  the  walls  are  made  impervious,  they  are 
uninjured  by  the  acids  of  the  silage. 

Concrete  therefore  makes  an  excellent  material  for  the  con- 
struction of  silos. 


TANKS,  CISTERNS,  AND  SILOS 


195 


Use  of  Concrete  for  Silos.  —  The  types  of  concrete  silos  in 
most  common  use  are: 


Fig.  86.  —  Concrete  Block  Silo. 

(1)  The  solid-wall  silo. 

(2)  The  hollow-wall  silo. 

(3)  The  concrete-block  silo. 

Of  these,  the  solid-wall  type  is  cheapest  and  most  generally 
desirable.  In  very  cold  climates  the  hollow-wall  or  the  block 
silo  is  desirable  to  prevent  the  freezing  of  the  silage.  Either  of 
the  first  two  classes  may  be  made  by  casting  the  concrete  in 


196     CONCRETE  CONSTRUCTION  FOR  RURAL  COMMUNITIES 

forms  or  by  plastering  it  on  metal  lath.  The  former  method  is 
more  commonly  used,  as  it  is  usually  cheaper,  and  does  not 
require  the  use  of  expert  labor.  Only  the  solid-wall  poured 
silo  will  be  described  in  this  book. 

Size  of  Silo  Required.  —  The  size  of  silo  required  depends  on 
the  size  of  the  herd,  the  length  of  time  it  is  to  be  fed  from  the 
silo,  and  the  amount  of  the  daily  ration.  The  silo  should  be 
of  such  diameter  that  at  least  two  inches  of  silage  will  be  used 
off  the  top  each  day.  Otherwise  the  top  layer,  which  is  exposed 
to  the  air,  will  mold  and  be  unfit  for  use.  The  height  should 
be  from  two  to  three  times  the  diameter.  The  latter  is  usually 
made  ten  to  twenty  feet.  Tables  XIII  and  XIV  will  be  found 
useful  in  choosing  the  size  for  any  given  case. 

Forms.  —  Several  patented  steel,  forms  have  been  brought 
out  for  silo  work.  They  are  a  great  convenience  when  many 
silos  are  to  be  built.  When  only  a  few  are  to  be  made  the  forms 
can  be  most  economically  made  of  wood  and  sheet  steel.  They 
are  usually  made  only  three  or  four  feet  high,  and  the  desired 
height  of  silo  is  obtained  by  alternately  filling  and  raising  them. 

The  forms  may  readily  be  constructed  by  the  method  described 
in  Chapter  V,  but  should  be  so  made  that  they  can  be  tightened 
and  loosened  by  wedges  or  by  bolts.  It  will  generally  be  pref- 
erable to  use  sheet  steel  for  the  outer  form,  this  being  stiffened 
either  by  wooden  ribs  or  by  angle  irons  bent  to  a  circular  shape. 

Construction  of  the  Silo.  —  The  silo  should  usually  extend 
about  four  feet  into  the  ground  or  as  much  more  as  is  necessary 
to  secure  a  solid  foundation  below  frost  line.  A  footing  of 
1:3:6  concrete,  two  feet  wide  and  one  foot  deep,  should  be 
placed  under  the  outside  wall,  to  distribute  the  heavy  weight 
over  the  soil  and  to  prevent  settling.  This  footing  should 
project  just  as  far  on  the  outside  of  the  wall  as  on  the  inside. 
In  firm  soil,  the  excavation  may  be  made  of  the  same  diameter 
as  the  silo,  and,  just  before  concreting  is  begun,  the  earth  may 
be  undercut  to  the  proper  diameter  for  the  footing.  In  loose  or 
sandy  soil,  the  walls  of  the  excavation  must  be  sloped  back 
to  prevent  caving.  The  floor  may  be  made  six  inches  thick, 
and  should  be  poured  with  the  footing. 


TANKS,  CISTERNS,  AND  SILOS 


197 


Table  XIII 

Size  of  Silo  Required 

(Based  on  a  ration  of  40  lb.  daily  for  each  animal.) 


Feed  for  180  days 

Feed  for  240  days 

No.  of 

i.    4> 

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W  a  o 

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3 

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2 

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82 
2*5  « 
o  eg 
082 

Tons 

Feet 

Feet 

Acres 

Tons 

Feet 

Feet 

Acres 

10 

36 

10 

25 

21 

^2 

48 

10 

31 

3£ 

12 

43 

10 

28 

3 

57 

10 

35 

4 

15 

54 

11 

29 

4 

72 

11 

36 

5 

20 

72 

12 

32 

5 

96 

12 

39 

6* 

25 

90 

13 

33 

6 

120 

13 

40 

8 

30 

108 

14 

34 

7* 

144 

15 

37 

10 

35 

126 

15 

34 

$h 

•  168 

16 

38 

11 

40 

144 

16 

35 

10 

192 

17 

39 

13 

45 

162 

16 

37 

11 

216 

18 

39 

14* 

50 

180 

17 

37 

12 

240 

19 

39 

16 

Table  XIV 
Approximate  Capacities  of  Round  Silos 


Inside  diameter  of  silo  in  feet  and  the  capacity  in  tons 

Height  of 

silo 

10  ft. 

12  ft. 

14  ft. 

10  ft. 

18  ft.               2( 

)ft. 

Feet 

Tons 

Tons 

Tons 

Tons 

Tons               T 

ons 

20 

26 

22 

30 

24 

34 

49 

26 

38 

55 

28 

42 

61 

83 

30 

47 

67 

91 

32 

51 

74 

100 

131 

34 

56 

80 

109 

143 

36 

61 

87 

118 

155 

196 

38 

66 

94 

128 

167 

212 

40 

70 

101 

138 

180 

229             2 

80 

198    CONCRETE  CONSTRUCTION  FOR  RURAL  COMMUNITIES 

The  inner  form  should  be  set  in  place  after  the  floor  is  poured, 
and  if  the  excavation  has  been  made  larger  than  the  silo  diam- 
eter, the  outer  one  must  be  placed  also.  The  steel  should  be 
wired  in  position,  and  the  forms  filled  with  1:2:4  concrete  mixed 
to  a  wet  consistency. 


Fig.  87.  —  Hoist  for  Use  in  Silo  Construction. 
This  hoist  is  operated  by  the  Mixer  Engine. 


The  next  morning  the  forms  should  be  loosened  and  raised 
nearly  to  the  top  of  the  concrete  already  placed,  care  being 
taken  not  to  injure  the  latter  by  striking  it  or  prying  against  it. 
The  forms  may  be  raised  by  means  of  jack  screws,  levers,  or  blocks 
and  tackles.  They  should  then  be  tightened  up,  the  reinforcing 
and  door  forms  placed,  and  placing  of  concrete  resumed.  This 
process  is  repeated  until  the  desired  height  of  silo  is  obtained. 


TANKS,  CISTERNS,  AND  SILOS 


199 


For  the  first  three  sections  the  concrete  can  be  shoveled 
directly  into  the  forms.  Above  this,  it  is  best  hoisted  in  buckets 
by  a  block  and  tackle,  drawn  by  a  horse  or  by  a  drum  on  the 
concrete  mixer. 


Ground  Line 


; 


23 


6> 


-2: 


fa 


TT2 


5'0- — >|< 1-  5'-0- 

I 


•■awM.-.-^A-jras 


Ground  Line 


Fig.  88.  —  Section  of  Silo,  Showing  Location  of  Doors. 

Upon  completion  of  each  section  of  the  silo  it  should  be 
given  two  brush  coats  of  neat  cement,  or  of  equal  parts  of  cement 
and  fine  sand,  mixed  with  water  to  a  creamy  consistency.  This 
closes  up  the  pores  in  the  surface,  and  helps  to  make  it  water- 
tight, besides  improving  the  appearance. 

Doors  and  Chute.  —  Two  types  of  door  are  used  in  silos,  that 
with  separate  openings^  and  that  with  continuous  openings. 
Separate  doors  may  be  made  2  feet  wide  by  2 J  feet  high,  and 


200    CONCRETE  CONSTRUCTION  FOR  RURAL  COMMUNITIES 


Fig.  89.  —  Arrangement  of  Reinforcing  at  Continuous  Door  of  Silo. 


Fig.  90.  —  Arrangement  of  Reinforcing  Around  Separate  Door  of  Silo. 


TANKS,  CISTERNS,  AND  SILOS  201 

spaced  3  feet  apart.  The  openings  may  readily  be  made  by 
placing  a  box  frame  of  the  desired  shape  between  the  inner  and 
outer  forms.  It  should  be  well  beveled,  so  that  it  can  easily 
be  withdrawn  after  the  forms  are  raised.  The  doors  may  be 
made  of  wood  cut  to  fill  the  opening.  They  should  be  drawn 
up  snugly  to  place  before  the  silo  is  filled,  as  they  must  be 
tight  to  prevent  the  silage  from  spoiling. 

Continuous  doors  are  somewhat  more  convenient.  In  these  an 
opening  about  two  feet  wide  is  left  from  the  roof  to  the  ground, 
spanned  only  by  reinforcing'  bars,  which  serve  also  as  a  ladder. 
Tongued  and  grooved  boards  laid  across  the  opening,  and  kept 
in  place  by  the  pressure  of  the  silage,  will  serve  to  close  the 
doorway. 

It  is  usually  desirable  to  have  a  chute  surrounding  the  silo 
doors.  It  may  be  made  about  three  feet  by  three  feet  in  sec- 
tion, and  may  be  fastened  to  the  silo  by  bolts  cemented  in  holes 
drilled  in  the  walls  or  left  there  for  this  purpose  when  the  silo 
was  built.  The  holes  may  be  made  by  means  of  taper  wooden 
plugs,  about  one  inch  in  diameter  at  the  smaller  end,  inserted 
in  the  concrete  when  it  is  poured.  Chutes  are  sometimes  built 
of  concrete  plastered  on  metal  lath,  or  poured  in  forms  con- 
structed for  the  purpose. 

Reinforcing.  —  The  hoop  reinforcing  for  silos  may  be  figured 
on  the  basis  of  pressure  from  a  liquid  weighing  11  lb.  per  cu. 
ft.  Use  the  method  described  in  Chapter  VII.  Some  vertical 
rods  should  be  provided  to  tie  the  whole  structure  together,  to 
resist  wind  stresses,  and  to  distribute  the  loads.  Tables  XV 
and  XVI,  from  Bulletin  No.  21  of  the  Association  of  Portland 
Cement  Manufacturers,  show  the  spacing  required  for  f-inch 
round  rods  and  for  various  sizes  of  wire. 

The  arrangement  of  the  reinforcing  at  the  doors  requires  careful 
attention.  At  least  as  much  steel  must  be  carried  across  a  ver- 
tical section  through  the  doors  as  at  any  other  section  of  the  silo. 
Figure  89  shows  a  satisfactory  arrangement  for  a  continuous 
door,  and  Fig.  90  for  separate  doors. 

Roofs.  —  The  roofs  of  silos  may  be  made  of  concrete  placed 
on  a  deep  rib  lath  or  reinforced  by  rods.  They  may  also  be 
made  of  wood,   covered  with  shingles.     The  latter  method  is 


202    CONCRETE  CONSTRUCTION  FOR  RURAL  COMMUNITIES 


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204    CONCRETE  CONSTRUCTION  FOR  RURAL  COMMUNITIES 

more  usual.     In  the  roof  a  window  or  door  should  be  left  through 
which  the  silo  may  be  filled. 


Fig.  91.  —  Framing  for  Roof  of  Silo. 

Cost  of  Silos.  —  The  cost  of  silos  varies  considerably  with 
local  conditions.  The  cost  of  materials  may  readily  be  calcu- 
lated for  any  given  set  of  conditions.  The  labor  cost  depends 
on  the  experience  and  ability  of  the  man  in  charge,  and  on  the 
apparatus  with  which  he  has  to  work.  One  contractor  in  west- 
ern Kansas  charges  the  following  prices  for  cement,  labor,  and 
apparatus,  the  farmer  furnishing  all  sand  and  gravel  delivered 
at  the  site  of  the  work,  and  boarding  the  crew. 

14  ft.  diameter $10  per  ft.  of  height 

16  ft.  diameter 11    "     "    "       " 

18  ft.  diameter 13    "     "    "       " 

20  ft.  diameter 15    "     "    "       " 

The  average  total  cost  for  materials  and  labor  per  ton  of 
capacity  is  given  by  the  Universal  Portland  Cement  Company 
in  the  following  table.  It  is  compiled  from  the  actual  costs  of 
a  large  number  of  silos  built  chiefly  in  the  states  of  the  upper 
Mississippi  Valley. 


TANKS,  CISTERNS,  AND  SILOS 

Table  XVII 
Average  Cost  of  Silos  per  Ton  of  Capacity 


205 


Monolithic 

Block 

Capacity  100  tons  or  less 

$2.89 
2.38 
2.18 

$3.52 

Capacity  100  to  200  tons 

2.88 

Capacity  over  200  tons    

3.11 

This  would  make  the  cost  of  a  12  ft.  by  30  ft.  monolithic  silo 
about  $200,  of  a  16  X  32  ft.  silo  about  $310,  and  of  a  20  X  40  ft. 
silo  about  $610. 


CHAPTER  XV 


SMALL  HIGHWAY  BRIDGES    AND    CULVERTS 

Use  of  Concrete  for  Bridges  and  Culverts.  —  Concrete  makes 
an  ideal  material  for  small  highway  bridges  and  culverts,  since 
when  they  are  well  made,  they  are  permanent,  and  the  cost, 
including  upkeep  charges,  is  usually  less  than  when  any  other 
material  is  used.  Mr.  A.  R.  Hirst,  Acting  State  Engineer  of  Wis- 
consin, gives  the  average  cost  of  several  types  of  small  culverts 

as  follows: 

Table  XVIII 

Cost  of  Maintaining  Small  Culverts  for  100  Years 


Kind 

Shape 

Size 

Cost 

Cost  for 
100  years 

Wooden  box 

Concrete  box 

Cast  iron     

Cast  iron     

Vitrified  tile 

Corrugated  steel.  . 
Concrete 

Rectangular 

Rectangular 

Semicircular 

Circular 

Circular 

Circular 

Circular 

15  "square 
15"  square 
16"  diameter 
18"  diameter 
18"  diameter 
18"  diameter 
18"  diameter 

$16.80 
40.00 
57.90 
92.40 
42.00 
50.40 
35.00 

$252.00 
40.00 
97.80 

166.80 
42.00 

196.00 
35.00 

This  shows  that  while  the  small  wooden  culvert  is  cheapest  in 
first  cost,  the  frequent  renewals  required  make  it  really  the 
most  expensive. 

In  larger  sizes,  it  is  often  found  that  the  first  cost  of  con- 
crete is  little  if  any  greater  than  of  the  light  steel  bridges 
frequently  used,  and  concrete  has  the  great  advantage  of  per- 
manence, with  no  cost  for  painting  or  repairs. 

Since  concrete  bridges  and  culverts  will  last  indefinitely,  they 
should  be  made  of  a  size  and  character  which  will  be  perma- 
nently satisfactory.  They  should  be  made  heavy  enough  and 
wide  enough   to   provide  for  future  increases  in  traffic;    they 

206 


SMALL  HIGHWAY  BRIDGES  AND  CULVERTS 


207 


should  be  placed  in  the  most  suitable  location;  and  they  should 
be  so  planned  that  the  roadways  may  be  practically  straight 
and  the  watercourses  direct.  Skew  bridges,  i.e.,  those  that 
cross  the  road  at  an  angle,  should  be  used  if  circumstances 
require  it,  but  the  ends  should  be  maintained  parallel  to  the 
line  of  the  road. 

Many  states  maintain  state  engineers,  or  state  highway  com- 
missions that  will  furnish  plans  and  specifications  and  will  su- 
pervise the  construction  of  bridges  and  culverts.  Their  advice 
and  assistance  should  be  sought  in  all  cases  of  importance. 


PR 

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Fig.  92.  —  Flat  Slab  Bridge,  20  ft.  Span. 

Types  of  Bridges.  —  The  three  principal  types  of  small  bridges 
are  flat-slab,  beam  or  girder,  and  arch.  Of  these,  the  flat-slab 
is  used  for  spans  up  to  twenty  feet,  and  the  beam  or  girder  for 
longer  spans.  The  arch  type  is  used  for  any  span  from  the 
smallest  culverts  to  bridges  over  great  rivers. 

The  arch  type  is  suitable  for  situations  in  which  there  is 
plenty  of  head  room,  as  over  deep  watercourses  in  hilly  country, 
where  bed  rock  foundations  may  be  had,  while  slab  or  beam 
bridges  are  more  suitable  where  the  head  room  is  limited,  or 


208     CONCRETE  CONSTRUCTION  FOR  RURAL  COMMUNITIES 

where  sand  or  clay  must  be  depended  upon  for  a  foundation, 
as  is  usually  the  case  in  flat  country.  The  artistic  appearance 
of  the  arch  will  sometimes  be  a  factor  affecting  the  choice  of 
the  type  to  be  used. 

Size  of  Bridge.  —  The  size  of  waterway  should  be  so  chosen 
that  it  will  carry  the  stream  at  any  time,  except,  perhaps,  dur- 
ing extreme  flood  conditions,  but  on  account  of  the  cost,  it 
should  be  no  larger  than  is  necessary.  It  is  a  common  fault  to 
make  the  waterway  too  small.  If  there  is  already  a  bridge  on 
the  site  or  near  by  on  the  same  stream,  observations  on  it  during 
high  water  will  aid  the  judgment.  If  there  is  none,  the  size 
may  be  determined  by  measuring  the  stream  at  some  narrow 
point  in  flood  conditions. 

Table  XIX  was  prepared  by  the  engineering  department  of 
the  Atchison,  Topeka,  and  Santa  Fe  Railway  for  areas  of  water- 
ways in  eastern  Kansas  and  Missouri  —  territory  which  has 
rather  steep  slopes  and  heavy  rainfalls.  It  is  based  on  a  six- 
inch  rainfall  in  twenty-four  hours,  most  of  it  falling  in  six  or 
eight  hours,  and  a  run-off  of  172  cubic  feet  per  second,  from 
each  square  mile,  with  a  velocity  of  four  miles  per  hour.  For 
flatter  sections  of  the  country,  or  those  with  less  violent  rains, 
the  areas  may  be  decreased  somewhat.  A  convenient  way 
to  remember  the  values  approximately  is  as  follows: 

For  drainage  areas  between  one  and  four  square  miles  allow 
one  hundred  square  feet  of  waterway  for  each  square  mile  drained. 
For  very  small  areas,  double  the  section  of  waterway  given  by 
the  rule,  and  for  large  areas,  decrease  the  values  somewhat. 

The  width  of  roadway  will  depend  to  some  extent  on  the 
region  in  which  the  bridge  is  built.  It  should  be  wide  enough 
so  that  teams  may  pass  readily  —  never  less  than  sixteen  feet, 
and  preferably  twenty  feet  or  more  where  there  is  much  traffic 
on  the  road. 

Foundations.  —  The  character  of  the  soil  at  the  site  of  the 
bridge  will  determine  the  size  and  character  of  footings  required. 
If  bed  rock  can  be  reached  it  will  carry  safely  any  load  likely 
to  be  placed  upon  it,  but  if  clay,  sand,  or  gravel  is  depended 
upon,  the  footings  must  be  made  wide  enough  to  reduce  the 
pressure  per  square  foot  to  a  safe  value. 


SMALL  HIGHWAY  BRIDGES  AND  CULVERTS 


209 


Table  XIX 

Area  of  Waterway  used  by  Santa  Fe  Railway  for  Bridges 
and  Culverts 


Drainage  area,  sq. 

Area  of  water- 

Drainage area, 

Area  of  water- 

miles 

way,  sq.  ft. 

sq.  miles 

way,  sq.  ft. 

0.01 

2.0 

0.50 

66 

0.02 

4.0 

1.0 

100 

0.04 

7.5 

2.0 

200 

0.08 

13.5 

4.0 

388 

0.15 

25 

8.0 

601 

0.25 

38 

15 

835 

Mr.  W.  S.  Gearhart,  State  Engineer  of  Kansas,  limits  the 
bearing  pressures  on  the  bridge  foundations  to  the  values  given 
in  the  following  table: 


Table  XX 
Safe  Bearing  Power  of  Soils 


Kind  of  material 

Safe  bearing  power 
in  tons  per  sq.  ft. 

Alluvial  soil 

\  to  1 

Earth  and  soft  clay 

1  to  2 

Clay  or  sandy  clay     

2 

Sand  or  gravel,  confined 

Cemented  gravel        

2 
5 

Rock,  cleaned  to  solid  bed .... 

up  to  25 

The  footings  should  be  placed  low  enough  to  be  below  frost 
line,  and  to  be  protected  against  being  undermined  by  the 
water. 

For  arches,  the  foundations  must  be  especially  secure,  since 
these  must  carry  heavy  thrusts,  and  a  slight  settlement  may 
cause  serious  consequences.  Large  arches  should  always  be 
supported  from  bed  rock,  if  possible. 

Abutments.  —  The  abutments  of  the  flat  types  of  bridges 
act  as  retaining  walls  to  hold  back  the  embankment,  and  as 
walls  to  support  the  bridge  floor.     Wing  walls  at  the  ends  of 


210     CONCRETE  CONSTRUCTION  FOR  RURAL  COMMUNITIES 

the  bridge  should  usually  make  an  angle  of  about  30  degrees 
with  the  waterway.  In  small  culverts  end  walls  are  often 
placed  parallel  to  the  roadway.  Weep  holes  should  be  provided 
through  the  abutments  and  wing  walls  to  prevent  the  accumu- 
lation of  water  behind  them.  In  cold  weather  the  freezing  of 
this  water  might  cause  serious  damage. 

Abutments  may  be  made  of  concrete,  plain  or  reinforced,  or 
of  stone  masonry  laid  in  cement  mortar.  It  will  frequently  be 
possible,  in  replacing  an  old  bridge,  to  use  the  abutments  already 
in  place,  but  this  should  not  be  done  unless  they  are  really 
suited  to  the  situation  and  are  built  properly  for  a  permanent 
structure.  Usually  plain  concrete  abutments,  with  a  little  steel 
to  tie  the  wings  and  end  walls  together,  will  be  found  preferable. 

The  height  of  the  abutments  depends  on  local  conditions,  and 
not  on  the  span.  They  should  always  be  carried  down  to  a 
solid  foundation. 

Flat-slab  Bridges.  —  Flat-slab  bridges  are  economical  up  to 
about  20  feet.  Tables  XXI  and  XXII  and  Figure  93  l  give 
dimensions,  amounts  of  materials  required,  and  details,  for  slab 
bridges  and  their  abutments,  for  spans  from  eight  to  twenty  feet, 
and  for  heights  of  abutments  from  five  to  ten  feet.  All  neces- 
sary data  are  given  in  the  figures  and  tables,  and  the  methods  of 
construction  to  be  followed  are  those  given  in  the  earlier  parts 
of  this  book.  The  width  of  the  footings  may  need  to  be  varied 
somewhat  to  correspond  to  the  character  of  the  soil,  using  the 
values  for  bearing  pressures  heretofore  given  in  this  chapter. 

Flat-top  Box  Culverts.  —  Flat-top  box  culverts  are  essentially 
small  flat-slab  bridges.  They  should  always  be  constructed 
with  wing  walls,  or  with  head  walls  parallel  to  the  roadway. 
They  should  be  placed  nearly  in  line  with  the  natural  direction 
of  stream  flow,  to  prevent  clogging  and  washouts.  The  length 
of  the  culvert  should  be  made  sufficient  to  give  a  24-foot  road- 
way whenever  possible. 

A  concrete  floor,  with  aprons  at  the  ends,  extended  as  low  as 
the  footing,  will  increase  the  capacity  of  the  waterway  and 
help  to  prevent  undermining  of  the  walls. 

1  Taken,  by  permission,  from  "  Small  Concrete  Bridges  and  Culverts," 
by  Universal  Portland  Cement  Company,  Chicagp. 


SMALL  HIGHWAY  BRIDGES  AND  CULVERTS 


211 


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SMALL  HIGHWAY  BRIDGES  AND  CULVERTS 


215 


Tables  XXIII,  XXIV  and  XXV  and  Figure  94  *  give  all 
data  necessary  for  the  construction  of  flat-top  box  culverts  for 
spans  of  three  to  eight  feet. 

Table  XXIII 

Dimensions  for  Wing  Walls  for  Flat-top  Culverts2 
Concrete:  1:  2i: 5 


Amount  of  materials,  including 

apron,  guard  rail,  and  floor 

Height 

Depth 

between  wing  walls 

Span 

+  floor 
thickness 

L  =  span 

G 

of 

apron 

Concrete, 

Cement 

Sand 

Gravel 

cu.  yds. 

bbls. 

cu.  yds. 

cu.  yds. 

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3'0" 

v  8" 

00  Z  60 

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1.6 

3.2 

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21.8 
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11.8 

16.0 
23.6 

1  Taken,  by  permission,  from  "Small  Concrete  Bridges  and  Culverts/ 
by  Universal  Portland  Cement  Company,  Chicago. 

2  See  Fig.  94. 


216     CONCRETE  CONSTRUCTION  FOR  RURAL  COMMUNITIES 


SMALL  HIGHWAY  BRIDGES  AND  CULVERTS 


217 


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220     CONCRETE  CONSTRUCTION  FOR  RURAL  COMMUNITIES 

Circular-top  Box  Culverts.  —  Small  box  culverts  can  be  eco- 
nomically built  with  circular  tops.  Figure  95  shows  details  of 
construction  of  such  culverts.     The  forms  are  made  of  two-by- 


Fig.  96.  —  Concrete  Arch  Bridge. 

fours,  beveled  and  strung  on  wires  as  shown,  enough  pieces  being 
used  to  give  the  size  desired.  They  are  then  rolled  about  cir- 
cular heads  and  tied  by  wires  on  the  outside.  Wedges  are  used 
to  make  the  form  tight.  After  the  concrete  has  hardened  suffi- 
ciently, the  wedges  and  heads  are  knocked  out,  and  the  forms 
are  removed.  Head  walls,  as  shown  in  the  illustration,  should 
be  used  with  these  culverts. 


INDEX 

[NUMBERS    REFER   to    pages] 


Aggregates,  34 

Alum    and   soap    for   waterproofing, 

144,  146 
Areas,  weights,  and  spacing  of  rods, 

113 

B 

Bank-run  gravel,  55 
Barn  floors,  182 
Beams  and  slabs,  103 

design  of,  116,  117,  119,  120 

effective  depth  of,  116 
Bituminous  shield  for  waterproofing, 

147 
Block  machines,  151 
Bonding  old  and  new  concrete,  84 
Brick,  concrete,  163 
Bridges,  highway,  206 

dimensions     and     reinforcement 
for,  211,  212,  213,  214 
Broken  stone,  40 

omission  of,  54 

screening  of,  41 

size  of,  41,  108 
Building  blocks,  150 

reinforcement  of,  157 

specifications  for,  155,  158,  162 

testing  of,  162 


Cement,  7 

care  of,  12 

choice  of  brand  of,  11 

kinds  of,  7 

methods  for  testing,  17,  32 

specifications  for,  13,  14 

use  of  the  word,  1,  7 
Cement  wash,  use  of,  84,  127,  129, 
139,   146,   190 

Cisterns,  192 
Coloring  concrete,  134,  140,  147,  148, 

154 
Columns,  107 

design  of,  115 

effective  size  of,  116 
Concrete,  definition  of,  1 

development  of  use  of,  7 

setting  of,  85 

strength  of,  86,  88,  111 

testing  of,  87 
Consistency  of  concrete,  71,  82,  109 
Corrosion  of  reinforcing  steel,  106 
Culverts,  206 

cost  of  different  kinds  of,  206 

circular  top,  dimensions  and  re- 
inforcement for,  217 

flat-top,    dimensions    and    rein- 
forcement for,  215,  216,  218, 
219 
Curing  of  concrete,  83,  86,  154,  157, 
180 


Capacity  of  silos,  197 
of  tanks,  190,  191 
Cellar  floors,  182 


Dairy-barn  floors,  182 
Deformed  bars,  99,  119 


222 


INDEX 


Depositing  concrete,  82 
under  water,  83 

Drain  tile,  165 

data  and  costs,  168 

E 

Expanded  metal,  100,  120 
lath,  100,  136,  138 


Feeding  floors,  183 

Fence  posts,  169 

corner  and  gate,  172 
reinforcement  of,  171,  172 

Floors,  182 

Forms,  62 

for  fence  posts,  169,  170 
pressure  of  concrete  on,  64 
removal  of,  70,  86,  110,  129 
tying  and  bracing  of,  64,  65 

Freezing  of  concrete,  89 

Fuller's  rule,  57 


Gravel,  screening  of,  42,  55 
washing  of,  43 

H 

Hand  mixing  of  concrete,  73,  75 
High  carbon  steel,  95 
Hollow  cylinders,  102,  108,  109 
design  of,  112,  114 


Impurities  in  sand,  38,  39 

J 

Joints,    contraction    and    expansion 
84,  180,  187 


Lime,  hydrated,  11 

manufacture  of,  10 

use  of,  129,  134,  137,  139,  143, 
144,  148,  189 
Lime  and  soap  for  waterproofing,  144 


M 


Measurement  of  materials,  75 
Measuring  boxes,  75 
Mechanical  analysis  of  sand,  37 
Mixers,  continuous,  78 

batch,  79 
Mixing  of  concrete,  71 
Mixing  platform,  74 
Mortal,  definition  of,  1 

N 

Natural  cement,  9 

definition  of,  9,  15 
production  of,  2,  7 
specifications  for,  15 
use  of,  10,  11 


Oil  for  waterproofing,  145 
Omission  of  coarse  aggregate,  54 


Painting  concrete  surfaces,  149 
Paraffine  for  waterproofing,  146 
Pipe,  reinforced  concrete,  167 
Placing  of  steel  in  columns,  107,  115 

in  continuous  beams,   104,   105, 
120 

in  hollow  cylinders,  102,   115 

in  simple  beams  and  slabs,  103, 
105,  119 
Portland  cement,  7 

definition  of,  7,  16 


INDEX 


223 


Portland  cement  {continued) 

invention  of,  2 

manufacture  of,  8 

production  of,  2,  7,  9 

specifications  for,  16 

use  of,  11 
Pressure  of  concrete  on  forms,  64 
Proportioning  concrete,  47 

methods  of,  51,  53 
Puzzolan  cement,  10 

manufacture  of,  1,  10 

production  of,  7 

use  of,  11 

Q 

Quantities  of  materials,  56 
by  Fuller's  rule,  57 
table  of,  59 


Reinforcing  steel,  specifications  for,  96 

Reinforcement  for  bridges,  211,  212, 
213,  214 
building  blocks,  157,  164 
culverts,  216,  217,  218,  219 
fence  posts,  171 
silos,  200,  201,  202,  203 
watertanks,  190,  191 

Removal  of  forms,  70,  86,  110,  129 

Re-rolled  steel,  98 

Roads,  concrete,  184 

standard  sections  for,  186 


Safe  stresses  in  steel,  112 

Sand,  35 

effect  of  impurities  in,  38 
requirements  for,  35,  40 
standard  Ottawa,  16,  24,  40 
testing  of,  39 
washing  of,  39,  43 

Screen  for  gravel  or  broken  stone,  41, 
42,43 

Selected  aggregates,  use  of,  128,  140, 
147 


Sidewalks,  177 

Silos,  193 

Soundness    of    cement,    14,    16,    17, 

30,  32 
Specifications  for 

building  blocks,  155,  158,  162 

cement,  13,  14 

oil  for  waterproofing,  145 

reinforcing  steel,  96 

stucco,  132 
Splicing  of  reinforcement,  102 
Standard  Ottawa  sand,  16,  24,  40 
Steel,  95 

Stone  screening,  use  of,  40 
Strength  of  concrete,  86,  88,  89,  111, 

161 
Stucco,  definition  of,  131 

specifications  for,  132 

surface  finishes,  132,  133,  139 
Surface  treatment  of  concrete,  125 

of  stucco,  132,  133,  139 

of  sidewalks,  180 
Sylvester's  wash,  146 


Tamping  of  concrete,  72,  82 
Thickness  of  block  walls,  158 
Tools  required  for  hand  mixing,  73 
Transporting  concrete,  81 
Twisted  square  bars,  100 


U 
Uniformity  coefficient,  36 


Voids,  47,  51 


W 


Water  glass  for  waterproofing,  147 
Waterproofing  of  stucco,  135 

of  concrete,  142,  143,  154 
Water  tanks,  188 
Woven  wire  for  reinforcing,  100,  120 


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